ovs-vswitchd: Allow bridge code to manage the database connection itself.

ovs-vswitchd: Allow bridge code to manage the database connection itself.

Until now, the ovs-vswitchd main loop has managed the connection to the
database.  This worked adequately until now, but upcoming patches will tie
the bridge code more tightly to the database, which means that the bridge
needs more control over interaction with the database connection and thus
that it is better for the bridge to handle that connection itself.  This
commit makes the latter change, moving the database interaction from the
ovs-vswitchd main loop into bridge.c.