Refactor: Remove RaftLogReader trait dependency from RaftLogStorage

[drmingdrmer]

Changelog

Refactor: Remove RaftLogReader trait dependency from RaftLogStorage

This commit removes the dependency of the RaftLogReader trait from the
RaftLogStorage interface in Openraft.

trait RaftLogStorage<C>: RaftLogReader<C> + ... becomes
trait RaftLogStorage<C>: ....

This change reflects the infrequent need for reading log entries
directly from storage, as it primarily occurs when applying logs to the
state machine.

Inside Openraft, logs can now be read by acquiring a reader via
RaftLogStorage.get_log_reader().await.xxx(), rather than requiring
RaftLogStorage to implement the RaftLogReader trait.

Upgrade(non-breaking) tip:

Implementations are advised (though it is not mandatory) to remove the
impl RaftLogReader for YourRaftLogStore from their codebase.

• Fix: Proposal: Remove RaftLogReader Trait Dependency from RaftLogStorage #1118

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This change is 

[schreter]

Makes sense, thanks.

BTW, I assume reading the log is effectively only used during the restart when we find out the state machine lags behind the last committed log, right? Then, the local log entries up to local log end are read and applied, remaining entries are then received from the leader via regular replication, written to log and applied then (which is no different from regular online operation).

I.e., the entries on the leader received from the client are kept in memory until they are applied (or thrown out when it ceases to be a leader) and similarly, entries received from the leader on the follower are kept in memory until they are applied on the follower.

Thus, the log is effectively never read, except for the restart case, right?

Reviewed 7 of 7 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @drmingdrmer)

[drmingdrmer]

Right.

For a Follower, the logs are read only when applying them. The RaftLogStorage implementation should keeps the logs in memory until applied. After that logs memory can be released.

For a Leader, logs are read when applying them and when replicating them to followers.
To support replication, the logs in memory should be kept for a longer time to maximize performance.

And logs are read when starting up.

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @drmingdrmer)