Optimize db.floating_ip_deallocate

'select for update' and 'update' operations
were combined into a single 'update' operation

problems with 'select for update' relate to
spec https://review.openstack.org/#/c/97310/1

now 'floating_ip_deallocate' function returns
number of updated rows in db

Partial-Bug: #1343613

Change-Id: I60f5a8f3e1541983dea1589783927107c00c5fa4
Merge "Optimize db.floating_ip_deallocate"