Support synchronize WAL in parallel #258
Description
Write to multiple WALs concurrently to leverage hardware parallelism.
Raft Engine provides a WriteToken that maps to a WAL stream underneath. If WriteToken is not provided, it will write to the default WAL stream.
User guarantees there will be no concurrent writing to the same region using different tokens. But writes to a region are allowed to be sent to different WALs at different time.
There're two ways to establish total order between logs among muliple log streams,
(1) Raft Engine have awareness of Raft term of the log entries. Key-values use the term of latest log in the region.
(2) Raft Engine internally manages a sequence number for each region.