Switch run-time import to using importutils.import_module

.. and enable the cyclic-import pylint check, now that this particular
import is invisible to pylint.

Change-Id: I9bfe7f77742b0db3ebead6a6767ade9b91e54c22
Catch DBReferenceError exception during binding a router

In some cases (Concurrent API tests) router can be deleted
before it is bound to an agent by scheduler.
That may lead to traces on L3 agent side returned via RPC.
Need to handle this case in more graceful way.

Change-Id: Ic52c26ace7f32a615150bd6098b244ae0b98250c
Closes-Bug: #1385257
Merge "Catch DBReferenceError exception during binding a router"
Merge "Switch run-time import to using importutils.import_module"