Test classes rather than equality in billing events

This works around MySQL and PostgreSQL adapters claiming that an object
is not in a collection even if all of the attributes are the same.
Fix the wrong droplet keys get generated.
Renamed key_from_sha1 to partitioned_key
Moving things around in blobstore test. 
Gradually remove the use of blobstore.files as it should not be exposed.
Renamed blob_store to blobstore

Also backfill test for StagingsController.store_droplet.
Remove sqlite3 build on Travis until we figure out the slowness
fix flaky death spec

there was a race between subscribing and killing the CC; if the CC was
killed first the message wouldn't be seen