The idea here is that transaction comments get copied to the ovsdb-server's
transaction log, which can then make it clear later why a particular change
was made to the database, to ease debugging.
An upcoming commit will add support for replicating tables across JSON-RPC
connection. As a prerequisite ovsdb itself must support basic replication.
This commit adds that support and then reimplements the ovsdb file storage
in terms of that replication.
