(#683) - _getAttachments called in generic adapter

This work moves getting attachments entirely to generic adapter leaving
_get of each adapter really simple and doing (almost) minimal work.

Reintroducted idea of txn in opts called now ctx. Usage here:

each adapter returns some ctx that is then passed to _getAttachment.
Using this we can ask about attachments in the same transaction as
get. This should be also used to ask about open_revs in the same
transaction.
(#712) - Make binary blobs work

For example PNGs weren't encoded properly into Blobs.
Now the roundtrip from base64 -> Blob -> base64 works
as expected.