Previously, `node.MuxRangefeed` created a child context for each rangefeed
request, storing it in the stream interface to allow the node level to be able
to shut down registration goroutines. This patch simplifies the approach by
passing the stream context directly to `p.Register`, eliminating the need to
store context in `streamSink` or return context via the interface. So this patch
also removes context from `kvpb.RangeFeedEventSink`.

Epic: none
Release note: none

This patch moves helper functions to registry_helpers_test.

Part of: cockroachdb#129814
Release note: none

…re tests

This patch refactors helper functions in registry_helpers_test.

Part of: cockroachdb#129814
Release note: none

# Conflicts:
#	pkg/kv/kvserver/rangefeed/registry_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/registry_test_helper.go

After the output loop completes, 2 cleanup steps are performed:

1) The registration is removed from the processors registry
2) The process is potentially stopped and removed from the replica.

Before this change, both of these happened syncronously after the
output loop finished.

With this change, step (1) happens asyncronously. To facilitate this,
an overflow mechanism is provided. This overflow mechanism potentially
allocates. Note that we expect that the number of requests is
relatively small and should be O(rangefeeds_on_range) so hopefully
this mechanism won't be used often.

Step (2) is now handled by the processor itself. After processing an
unregister request, if the set of registrations falls to zero, we
enqueue a Stop event for ourselves. Then, when processing the Stop, we
unregister ourselves from the replica.

Note that this may look like a small semantics change since the
previous unregister callback called Stop() which processes all events.
However, note that a Stopped event is processed after all other
events, so any events in the queue at the point of processing the
unregistration that enqueued the stop will be processed.

The motivation for this change is to eventually allow cleanup step (1)
to be run as part of registration.disconnect(), which needs to be
non-blocking. This is desired for a future change in which there is
not a dedicated goroutine to perform this cleanup.

Epic: none
Release note: None

This patch adds unbufferedRegistration.

UnbufferedRegistration is like BufferedRangefeed but uses BufferedStream to
buffer live raft updates instead of a using buf channel and having a
dedicated per-range per-registration goroutine to volley events to underlying
grpc stream. Instead, there is only one BufferedStream for each incoming
node.MuxRangefeed rpc call. BufferedStream is responsible for buffering and
sending its updates to the underlying grpc stream in a dedicated goroutine
O(node).

Note that BufferedStreamSender is still left unimplemented. This commit doesn't
add any tests for it yet. More tests will be added in future commits of this PR.

Part of: cockroachdb#129814
Release note: none

# Conflicts:
#	pkg/kv/kvserver/rangefeed/scheduled_processor.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/scheduled_processor.go

This patch adds a new data structure eventQueue which is like a queue but uses a
fixed size for the chunked linked list. Each chunk has a fixed size of 4096
elements. This implementation uses sync.Pool to reduce the number of
allocations.

pushBack, popFront, len run in constant time. removeAll runs in linear time with
respect to the number of elements in the queue. This structure is not safe for
concurrent use.

This is for future commits to include the queue in the BufferedSender to buffer
events at the node level.

Part of: cockroachdb#129813
Release note: none

This patch is the last step for reducing long-running O(ranges) goroutines in
kvserver/rangefeed. It changes the BufferedSender to use a queue to buffer
events before forwarding them to underlying grpc stream.

Closed: cockroachdb#129813
Release note: A new cluster setting
`kv.rangefeed.buffered_stream_sender.enabled` can now be used to allow rangefeed
to use buffered sender for all rangefeed feeds instead of buffering events
separately per client per range.

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender.go

This patch refactors the error for kvpb.RangeFeedRetryError_REASON_SLOW_CONSUMER
into newRetryErrBufferCapacityExceeded.

Part of: cockroachdb#126560
Release note: none

This patch adds capacity to node level buffered sender which will shut down all
registrations if the node level buffer had overflowed.

Part of: cockroachdb#129813
Release note: none

TODO: add a larger test at the kvclient side to make sure error returned here is treated as a restart signal

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender.go
#	pkg/kv/kvserver/rangefeed/buffered_sender_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/registry_test_helper.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/processor_test.go
#	pkg/kv/kvserver/rangefeed/registry_helpers_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/processor_helpers_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/processor_test.go

This patch adds more test cases for unbufferedRegistration.

Closed: cockroachdb#126560
Release note: none

# Conflicts:
#	pkg/kv/kvserver/rangefeed/registry_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/registry_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender_test.go
#	pkg/kv/kvserver/rangefeed/unbuffered_registration_test.go

# Conflicts:
#	pkg/kv/kvserver/rangefeed/buffered_sender_test.go