stateful design

[michielbdejong]

So far, ocm-stub was really just a stub that gives politically correct answers.
We now want to use it to test and develop new evolutions of the OCM protocol.

So:

• login/logout state should work (but it can be fake, with a session id in the query param)
• the server should generate a keypair for httpsig on startup
• it should remember sent and received shares and their state
• it should probably also remember webdav bearer tokens it gave out

It's fine if this state is just in memory, doesn't need to be on disk or in db.