MB-100: _async_load_all_buckets with batch_size=1K in xdcr

Change-Id: I6e1bd75a71c5fa46a2174e61157e71119d2c6bfd
Reviewed-on: http://review.couchbase.org/24668
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>