Conflux v2.2.4-testnet

RPC Changes

• Implement trace_epoch in the core space RPC endpoint to return traces of two spaces in an epoch.
• Update the Log data format returned by cfx filter RPCs.

Configurations

• Add keep_snapshot_before_stable_checkpoint to decide whether to keep the snapshot before the era checkpoint. Setting it to false would reduce the disk space usage (it's still default to true for further testing).

Improvements

• Avoid having multiple broadcast tasks for the same peer.