MB-11010: reduce lock contention in processBufferedItems()

In processBufferedItems(), responses are processed one at a time
from a particular stream's buffer. This leads to a lot of
contention on the streamMutex protecting the stream. Reduce the
contention by batch-processing the responses. Currently the batch
size is arbitrarily set to 10.

Change-Id: If42e0e1aa208be381e116f78293ef749e1509143
Reviewed-on: http://review.couchbase.org/37568
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>