cdb performance improvements in vizd

1. In cdbq dequeue callback accumulate the mutations to be written
   to cassandra in mutation map and then do batch write to cassandra
   from cdbq task exit callback. This improves cdbq cassandra write
   performance since we are bunching kMaxIterations write. Cdbq can
   now handle database writes corresponding to around 2K sandesh
   messages/sec.
2. Changed GenDb::NewCol to contain pointer to DbDataValueVec to
   minimize copying
3. Changed GenDb::ColList to contain ptr_vector of NewCol instead
   to vector to minimize copying
4. Add watermarks for SandeshStateMachine work queue to prevent
   vizd running out of memory
5. Scope SandeshRole enum to prevent clash with classes Generator
   and Collector in vizd
6. Fix memory leak in CdbIfQueue in cases when Generator disconnects
   and the queue contains items, introduced via commit
   f2524f9c64ebf69116065a39794890ea3cc739f0