context

we are slowly but surely bringing signalc up to feature parity with signald (for the subset of features needed by signalboost)
this MR implements the unsubscribe command, and sets us up to play #473 (closed) (in which we will need to unsubscribe from channels as part of gracefully handling restarts)
as a bonus: we also make it impossible to subscribe to messages for the same account twice (#425 (closed))

changes

add socket response types for unsubscribe success & failure
add new username accessor on requests (to make it easier to call this property on an arbitrary request)

modify SignalReceiver#subscribe:
- add subscription jobs to a concurrent hash map as created (so they can be retrieved later for cancelation)
- don't create a new subscription if one already exists for a given account
add SignalReceiver#unsubscribe:
- shuts down the signal service message pipe
- cancels the listening job (with a custom exception to signal to the original subscribe caller that it was intentionally cancelled and not unintentionally disrupted due to -- eg -- a broken connection to the signal server)
- removes all resources particular to this account from hash map cashes (eg: message sender, message pipe, subscribe job, cipher)
modify SignalRecever#rejectUnverified to be more generalizable, and thus suitable for use in both #subscribe and #unsubscribe

implement SocketReceiver#unsubscribe: handle subscribe socket requests, forward them to signal layer and signal success/error back to client
modify SocketReceiver#subscribe to distinguish a subscribe job cancelled on purpose from one cancelled due to connection disruption and not attempt to resubscribe in the former case