While parsing the configuration file, if a key doesnt exist we need to
skip it.

All batching code has no effect by default with this commit. Since we
know that this is a dangerous new feature we will enable them only on
our test servers and check them manually before setting it as default
or adding more configuration features.
Use SyncTarget and server conf file to enable it for testing.

Batch support is optional. This commit adds a 'batching' configuration
option to disable it.

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.

Creating a resource from a path to use get_json causes a lot of dirty
code and unexplained things like response[2]. This commit extracts that
logic into a helper to let it more clear about what is happening.

Added tests for this token verification as it wasn't covered. Then moved
it to the new couch module that implements a couch storage.
The ServerState was chosen to hold the verify_token method.
CouchServerState holds the current implementation, which is called on
authentication middleware as the new test shows.

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.