(#2931) - parallel compaction keeps db integrity

Contains two fixes:

1. adapter.js no longer passes the rev_tree to the underlying
   adapter. The adapter is expected to recalculate, since other-
   wise the database may change in the interim.
2. leveldb.js has a global write lock for all readwrite operations,
   similar to how WebSQL/IndexedDB work under the hood. I'm not
   100% satisfied with that, but it's better for the db to be
   correct than to be fast.
(#2185) - Remove allowed failures
(#2957) - Add appcache manifest
(#2951) - fix concurrency in leveldb.js

So I figured out where the bug was coming from - I had
not implemented the writeLock correctly. Took this opportunity
to also use the `double-ended-queue` as suggested by
@calvinmetcalf.