We have been discussing about this merge for a while.
Its main goal is to simplify things: code navigation, but also
packaging.

The rationale is that the code is more cohesive in this way, and there's
only one source package to install.

Dependencies that are only for the server or the client will not be
installed by default, and they are expected to be provided by the
environment. There are setuptools extras defined for the client and the
server.

Debianization is still expected to split the single source package into
3 binaries.

Another avantage is that the documentation can now install a single
package with a single step, and therefore include the docstrings into
the generated docs.

- Resolves: #8896

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.