added mock nats class

Change-Id: I6c5f5e812c73dc6edbce66abb8285f7cfb971046
added multi response requester to work with nats

Change-Id: I024a3e5859643a36d16c6c2c7082dc644aec7503
update AppStager to pass in async flag to stagers

Change-Id: I7ea7b9ce059d861997d9badcc6fa25ec48e77942
defer heavy lifting of storing droplet when finishing staging

Change-Id: I85370401080c3684e89c67483d7ac40f082a32c9
do not use VCAP::Stager::Client::Error in MultiResponseNatsRequest

Change-Id: I1bdf171567189f43687a98921509aa732924ce77
return timed out error to staging response handlers

Change-Id: I458df638f2b8225140a3112cf1428e6e6cd50e7c
be more paranoid in MultiResponseNatsRequest specs

Change-Id: Icb4f732dd24f24bfd4d6c5d55f99e044e999f15d
serialize and deserialize messages going thru MultiResponseNatsRequest
in the same place

Change-Id: I5ee5450a3575d3f735c047bc76e42c7518505c55
added MultiResponseNatsRequest instrumentation

Change-Id: Ie5879db209e41ea45577ab3f1bdadc25d6959ecb