database: rename clearSnapshot() => flushSnapshot()

database: rename clearSnapshot() => flushSnapshot()

* This is more consistent with the name flushReplicaSnapshots().
* Also add it to IDatabase, which defines the modern OLTP methods
  of DatabaseBase. This methods is useful for callers that get
  locks or wait for a replica DB to catch up before querying it.

Change-Id: Ia31e480bb9ccf461bf05ede4278920918eec4f16