(#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.

---

I'm not yet sure how far should I continue this work of abstracting logic into generic adapter. If sth like:
_getMetadata and then _getDocument is ok? (and implement all logic in generic adapter)
I'm not sure if we can do the same with bulkDocs.