CBIDXT-111: fixing race condition.

Race exists in accessing feed.vbstreams map[vbno]UprStream structure.
Fixed it using mutex-lock. In case it has an impact on performance we can take
the non-trivial activity of merging control-path and data path.

Patchset-2:
- Implemented Manik's comments.

Change-Id: I0eba4a5ed66072d40655b7249b23b3439efce231