Support async request
Support async request

Change-Id: I03675b7ead952c2af9b5caad825f6ee4fbd37d07
Reviewed-on: http://review.couchbase.org/45726
Reviewed-by: John Liang <e1e1383402f9cdee00b021b8a653005f3061b036@gmail.com>
Tested-by: John Liang <e1e1383402f9cdee00b021b8a653005f3061b036@gmail.com>
Reviewed-on: http://review.couchbase.org/46203
Reviewed-by: Sriram Melkote <3995a3476470ce2c0c01f75cee639e7d47357a62@couchbase.com>
Tested-by: Sriram Melkote <3995a3476470ce2c0c01f75cee639e7d47357a62@couchbase.com>

t3rm1n4l/gometa,couchbase/gometa