[5.0] Support throttling block syncing to peers

[jgiszczak]

Peer to peer listen ports may now apply a block sync throttle. Each occurrence of p2p-listen-endpoint has an independent throttle specification. Here's the updated help text:

--p2p-listen-endpoint arg (=0.0.0.0:9876:0)
                                        The actual host:port[:<rate-cap>] used
                                        to listen for incoming p2p connections.
                                        May be used multiple times.   The
                                        optional rate cap will limit block sync
                                        bandwidth to the specified rate.  A
                                        number alone will be interpreted as
                                        bytes per second.  The number may be
                                        suffixed with units.  Supported units
                                        are:   'B/s', 'KB/s', 'MB/s, 'GB/s',
                                        'TB/s', 'KiB/s', 'MiB/s', 'GiB/s',
                                        'TiB/s'.  Transactions and blocks
                                        outside of sync mode are not throttled.
                                          Examples:
                                            192.168.0.100:9876:1MiB/s
                                            node.eos.io:9876:1512KB/s
                                            node.eos.io:9876:0.5GB/s
                                            [2001:db8:85a3:8d3:1319:8a2e:370:7348]:9876:250KB/s

Parsing of the rate cap accepts fractional numbers expressed as decimals. The throttle rate is in bytes per second. Per the help text, shorthand suffixes are supported, and transactions and blocks being propagated across the peer to peer network between synchronized nodes are not throttled. Only blocks transmitted to a syncing node are throttled. A throttle of 0 or 0B/s means unthrottled, and is the default.

The limit for specifying a rate cap is 30 characters, including the colon field separator. This is sufficient characters to specify any value in the 64 bit range of size_t with room left for a suffix. If the rate cap does not parse or the value multiplied by the suffix exceeds 18,446,744,073,709,551,615 an exception will be thrown and the node will not start.

IPv6 addresses are supported for listen endpoints. They must be in square bracket format: [<ipv6 address>]:port optionally followed by :<rate-cap>.

Note

Inbound connections from IPs which are configured p2p-peer-addresses are exempt from throttle rate caps. Care should be taken in NAT environments to avoid inadvertently exempting connections due to overlapping subnets.

Throttling is stable even at exceptionally low byterates, but of course on a busy network if the throttle is less than the average block size on the network, clients using that listen port will never catch up to the head block. Such configurations are allowed but not recommended.

Suggested network topology

Together with #1411 which allows multiple listen endpoints, an edge node for public peering might be configured as follows:

Resolves #1295.

Retargetting #1540 to release/5.0.