NCBC-278: Lock WebClient when getting WebRequest

WebClient is not thread safe

Change-Id: I92e6aa78d58a37ee41e02d516a836defc17efa6f
Reviewed-on: http://review.couchbase.org/27355
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Saakshi Manocha <saakshi.manocha@globallogic.com>
Tested-by: Saakshi Manocha <saakshi.manocha@globallogic.com>