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
Merge "Removing rpc communication from db transaction"