Enable wal_flush_before_commit by default.

Enabling wal_flush_before_commit by default allows us to prevent the memory
bloating issue when the client never or rarely issues a commit.

However, this change disables wal_flush_before_commit in a compactor to reduce
the lock contention with the writer thread.

Change-Id: Ib3cd9e2ec56b151cb32db178f75cd6e5fdd396e1