ovsdb: Implement a "lock" feature in the database protocol.

ovsdb: Implement a "lock" feature in the database protocol.

This provides clients a way to coordinate their access to the database.
This is a voluntary, not mandatory, locking protocols, that is, clients
are not prevented from modifying the database unless they cooperate with
the locking protocol.  It is also not related to any of the ACID properties
of database transactions.  It is strictly a way for clients to coordinate
among themselves.

The following commit will introduce one user.