First step of splitting classes across files on common.
backend.py holds SoledadBackend (generic backend logic)
couch/ is now a directory with old code inside __init__.py and
CouchServerState on state.py
Also removed mock IndexedSoledadBackend, since Soledad does not support
indexing due to encryption on server side.
Also fixed DesignDocUnknownError to show up what is the message of the
original exception. It was being lost.

CouchDatabase was renamed to SoledadBackend and a new class
CouchDatabase was created to hold all couchdb code. This should make
SoledadBackend less tied to database implementation. A few more
separations are needed to split into modules.

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.

LEAP Platform needs to granularly allow access on user database for
other services, like mx. This is now possible by editing
soledad-server.conf file. A new section 'database-security' was added
and it is parsed during 'create-user-db' to be set on security design
document, present on every per-user database.