Add support for hash key

Keys may now be objects that look like:
{ key: 'key', hashkey: 'hashkey' }

Change-Id: Ifc8fff9872903996daf0b596f1c5fda46a8ab24c
Reviewed-on: http://review.couchbase.org/23933
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>