Removing rpc communication from db transaction

In a transaction context of ExtraRoute_db_mixin.update_router() was
called super method that uses subtransaction and communication with l3
agent. In case of agent's heartbeat happens while there is running
transaction, update heartbeat in agents table gets stuck in a deadlock.

Closes-Bug: #1211915
Change-Id: I96e6a9d7172d5a0e3e720a81fcd10f04c40aef07
(cherry picked from commit 5becbbec6a686667925f399904cebd97b583c35a)