ovsdb-idl: Break out database-specific stuff into new data structure.

ovsdb-idl: Break out database-specific stuff into new data structure.

Until now, a given ovsdb-idl instances has only monitored a single
database.  In an upcoming commit, it will grow to also monitor a second
database that represents the state of the database server itself.  Much of
the work is the same for both databases, so this commit breaks the common
code and data out into new data structures and functions.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Justin Pettit <jpettit@ovn.org>