[refactor] split out backend from couch database

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.

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