MB-100: add async load from cluster to load_memory test

Change-Id: I73be643aee2f06badd2d4230ee9800b2ceb95973
Reviewed-on: http://review.couchbase.org/25149
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>