(#2818) - compaction removes orphaned attachments

Migrated idb/websql to have a new many-to-many table
of seqs to digests, fixed compaction of attachments
in all three adapters, added many new tests.
(#2926) - remove vendor from cordova app
(#2923) - add blob-util to docs
(#2918) - revoke object url for test blob
(#2890) - reduce md5 chunk size to 32768
(#2913) - parallel compaction works