1. Message and message index table db writes were happening without going
   through rate limits set for db queue and hence contrail-collector would
   end up having huge db queues and high memory usage when
   contrail-vrouter-agent was sending AgentRouteLog in a loop due to
   route flaps/another unrelated bug. libstdc++ default allocator
   does not release memory back to the OS and hence contrail-collector
   would seem to have a memory leak due to the above scenario.
2. Add rates limits in sandesh client state machine and in collector per
   generator state machine and db queues even for SYS_EMERG messages.
3. Reset the db handler drop level on initializing also to handle
   reinitialization scenarios.

Change-Id: I9a26f414994eaea7af0ce9078839deb1dd391656
Closes-Bug: #1391757
(cherry picked from commit 9313a043912389f5e5e2f11a102b430afcfa445a)