L3 RPC loop could delete a router on concurrent update

routers_updated does not acquire any lock just updates
a set for future rpc loop processing.

The self.updated_routers can be changed by concurrent update
notification. If this change happens at the time around the
self.plugin_rpc.get_routers call, the additional routers
- by mistake - is considered as admin_state_up=false routers, which
 are safe to delete.

Creating a local copy of the updated_routers and preserve
the fresh updated_routers entries for the next _rpc_loop
operations.

Change-Id: Icc7377f9c29e248c3b34562465e859b15ecc2ec3
Closes-Bug: #1315467
Partial-Bug: #1253896
(cherry picked from commit 45381fe1c742c75773d97f1c0bd1f3cb1e7a6468)
Merge "L3 RPC loop could delete a router on concurrent update" into stable/icehouse