Race condition of L3-agent to add/remove routers

Race condition of L3-agent to add/remove routers

This race condition happens when repeatedly calling
l3-agent-router-add and l3-agent-router-remove
by different neutron-servers at the same time.

The primary key constraint is added for the pair of
(router_id and l3_agent_id).

During migration, verification is done if the current
records violate the PK constraint defined in this bug
fix, and sanitize the data before schema modification.

Due to different dialects of database engines, different
sql statements are executed correspondingly to do
the verification.

Change-Id: Ia541e023b757b2e77c4eec9bb1670632c7a271fa
Closes-Bug: #1230323