Created two methods on the backend to start and finish a batch. A dict of
callbacks is available to defer actions for the last document, allowing
temporary (changing often) metadata to be recorded only once.
Using those methods we will also be able to put all docs in one go on
the CouchDatabase implementation, but that is another step.

This was added for a Twisted 12 bug that should be gone by now.

This parameter applies the ensure_database to all existing databases,
making all of them use the latest design documents.
This can be used to migrate security documents, update handlers or any
other kind of design document.

- Resolves: #7509

netrc file was hardcoded inside create-user-db. Now it reads the path
from /etc/leap/soledad-server.conf as done on server process.
The new configuration property is called 'admin_netrc'.

README with information about latest change, missing docs and licenses,
variable naming and pep8.

From:
[refactor] removing getters and setters from couch.py
_couch_url was a private variable with getter and setter doing the same
as a public variable. It is accessed all over the code, so being private
with getters and setters didnt make sense. This commit fixes the tests
to also follow this style from now on.