(#2863) - Test we cant modify existing revision
(#2863) - Early return if bulkDocs tries to rewrite existing revision
(#2861) - Use a query to find update_seq instead of manually tracking it
(#2880) - fix base64-encoded attachments in http.js

The http adapter is not correctly handling base64-encoded
strings in putAttachment(). Instead of correctly recognizing
the text as base64, it actually just stores it as-is.

This also adds a test to ensure that we throw an error
if the user supplies a string that is not base64-encoded.
(#2867) - do not stack notify change listeners
Revert "(#2867) - do not stack notify change listeners"

This reverts commit d9429d1587dd0c50a963972f79c730389330ded2.
(#2880) - fix typo in comment/test name
(#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.