Add support for ACI/PNI

[gferon]

Since we don't support PNI yet, we might want to ACK the messages we receive to avoid seeing them in the loop all the time.

Maybe this is a bad idea and we should spend effort implementing it instead.

[rubdos]

We should probably start implementing the interface instead of just ignoring messages.

[gferon]

Note: basic support for making sure we have a PNI UUID is in place, but we currently hardcode the use of ServiceIdType::AccountIdentity in a bunch of helper methods like update_pre_key_bundle. Fully supporting PNI requires having two separate implementations of the ProtocolStore that end up storing a separate key-pair, sessions as well as pre-keys.

@rubdos I think a good way to do this would be to add our own ProtocolStore trait (or AccountStore?) that has two methods: aci_protocol_store and pni_protocol_store. That way, we could have functions only take a ServiceIdType as a parameter, and mimick what is done in Signal-Android. Design wise, we can really think of whatever works best (and helps us avoid adding more type parameters).