CCBC-459: Add error categories for server load and server responses

This commit adds two new error classifiers:

- LCB_ERRTYPE_SRVGEN for replies which are mapped one-to-one with a
  server reply.
- LCB_ERRTYPE_SRVLOAD for replies which unequivocally indicate the
  server is under load.

Change-Id: I7d5bc6a2ff13dcbc4fa8f3d4ebbe1f140f123355
Reviewed-on: http://review.couchbase.org/38839
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>