[API] Fix race in load balancer deletion

A PENDING_DELETE status may not be written to the DB before the gearman
thread executes.  This means an exception is fired in send_delete().

This patch causes the DB data to be committed before the gearman thread
is executed.  It also makes sure that the remaining LBs don't include the
one getting deleted.

Change-Id: I17618af2709535c525ff99bd03958891d61f5624