CCBC-407: Increase performance on select

- fd_set is itself a bit array. Do not allocate an array of these
- select should use the maximum fd +1 as its first argument, not
  FD_SETSIZE

Change-Id: Id1d8fc199b15eba5ffe720cf483fb9a748b6e21b
Reviewed-on: http://review.couchbase.org/36758
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>