Large commit which removes knowledge from the server. Ultimately the server
should only handle opaque (encrypted) blobs.

Ocsigen-start user accounts are removed. They will be done in a very
different manner (see #6).

Server-side handling of deltas is removed. Clients send their consensus
once in a while for the server to act as a cache.

As a consequence, exports are disabled for now (they were done
server-side).

Data is stored in an sqlite database. One table per pad. Might change in
the future (see #41).

Uses of Queue have been replaced with CCFQueue from Containers (see #8).