ovsdb-server: Store databases in shash instead of array.

An upcoming commit provides the ability to add and remove databases.
Having the databases in a shash instead of an array makes it easier
to add and remove databases.

Feature #14595.
Acked-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>