ovn-controller: Avoid blocking to commit OVSDB transactions.

ovn-controller: Avoid blocking to commit OVSDB transactions.

Until now, ovn-controller has been full of loops that commit a transaction
to the OVS or OVN Southbound database.  These blocking loops delay other
work within ovn-controller.  They also make it unsafe to keep pointers to
database records within a single ovn-controller main loop, since calls
to ovsdb_idl_run() can cause IDL records to be destroyed.  This commit
drops all of the blocking calls, instead doing a single commit to the
databases at the end of each main loop.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Russell Bryant <rbryant@redhat.com>