ovsdb-server: Add support for a built-in _Server database.

ovsdb-server: Add support for a built-in _Server database.

The _Server database is valuable primarily because it provides database
clients a way to find out the details of changes to databases, schemas,
etc. in a granular, natural way.  Until now, the only way that the server
could notify clients about these kinds of changes was to close the session;
when the client reconnects, it is expected to reassess the server's state.
One way to provide this kind of granular information would be to add
specific JSON-RPC requests to obtain notifications for different kinds of
changes, but since ovsdb-server already provides granular and flexible
notification support for databases, using a database for the purpose is
convenient and avoids duplicating functionality.

Initially this database only reports databases' names and schemas, but
when clustering support is added in a later commit it will also report
important aspects of clustering and cluster status.  Thus, this database
also reduces the need to add JSON-RPC calls to retrieve information about
new features.

Signed-off-by: Ben Pfaff <blp@ovn.org>