Add string cntl for http poolsize

Change-Id: Ie70e84f89098a545194e728a9ea000bee1e58802
Reviewed-on: http://review.couchbase.org/42166
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>