Feature/soledad adbapi3

DO NOT MERGE YET
just for testing / pullreq purposes

It marks the document as deleted. More specifically, it:

* fetches current version of doc stored in the db.
* raises if fetched doc is `None`, or revisions don't match, or doc is already deleted, or doc has conflicts.
* allocates a new revision.
* marks the document as tombstone.
* puts the tombstone doc into the db.