NewEtcdSyncer creates a new Syncer.

NewMemSyncer creates a new memory Syncer.

InitialVersion is the initial schema version for every server.

CheckVersFirstWaitTime is a waitting time before the owner checks all the servers of the schema version, and it's an exported variable for testing.

MemSyncer is in memory schema version syncer, used for uni-store where there is only 1 TiDB instance.

Syncer is used to synchronize schema version between the DDL owner and follower.