JCBC-461: Limit the netty worker count for streaming conns.

Motivation
----------
Under normal circumstances, netty should create worker threads on the fly
and since the client only uses one channel for the bucket streaming connection,
there should every only one be used.

Under failure conditions, it could be the case that many more threads
are created falsely.

Modifications
-------------
The worker count is held to one thread only, since only one channel needs
ever to be accessed.

Result
------
Providing a hard upper limit for worker threads prevents thread exhaustion.

Change-Id: Idb285bc794c190b876e63383d1eff690ba57d9ba
Reviewed-on: http://review.couchbase.org/37722
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>