lcbio: optimize field layout and access patterns

- Move some fields closer together in lbcio_CTX
- Use cached fd field in CTX rather than ->sock. This makes it so that
  we never actually have to reference the socket structure in normal
  usage

Change-Id: Idd8cfbf7d4ec0cedb8f70dee1745bbb8bc263218
Reviewed-on: http://review.couchbase.org/38349
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>