bridge: Make configuration database records valid all the time.

bridge: Make configuration database records valid all the time.

Before, it was possible for records in the configuration database to
disappear, so all of the ovsrec pointers inside bridge structures had
comments cautioning against their use except during reconfiguration.  But
now that the bridge has direct control over when ovsdb_idl_run() is called,
it can ensure that bridge_reconfigure() is always called immediately
whenever the IDL data structures change.  That means that we can use the
ovsrec configuration at any time after the reconfiguration process
initializes them, not just during reconfiguration.