Remove SELECT FOR UPDATE use in ML2 type driver release_segment

Remove SELECT FOR UPDATE use in ML2 type driver release_segment

SELECT FOR UPDATE expression, which is triggered with the use of the
SQLAlchemy Query object's with_lockmode('update') method, is
detrimental to performance and scalability of the database
performance code in Neutron due to the lock contention it produces.

SELECT FOR UPDATE can be entirely avoided in release_segment methods
with the use of single-shot UPDATE and DELETE expressions, and this
patch clears a number of uses of SELECT FOR UPDATE by consolidating
multiple SQL expressions into one.

Partial-Bug: #1330562
Change-Id: I29ffcafc8d4d73ac1cb50c94df5da85514d47a3f