Replace None with Some("") for state_key

The only situation that isn't working yet is sending `left` events for
users when the sender leaves the room

This fixes the panic from ruma "index out of bounds"

Move all state event appending to append_state_pdu.

feat: first steps towards joining rooms over federation
Add state-res as a dependency of conduit
Add reverse_topological_power_sort before append_pdu
Implement statehashstatid_pduid tree for keeping track of state
Clean up implementation of state_hash as key for tracking state

Reviewed-on: https://git.koesters.xyz/timo/conduit/pulls/181

The only situation that isn't working yet is sending `left` events for
users when the sender leaves the room

Merge pull request 'Make default server name more obvious' (#192) from spaetz/conduit:master into master

Reviewed-on: https://git.koesters.xyz/timo/conduit/pulls/192


Reviewed-by: Timo Kösters <timo@koesters.xyz>

Too many people fell into the trap and left conduit.rs in the deployment
configuration, wondering why users are all listed as @conduit.rs.

Make the default server name really easy to identify and make it obvious
that this needs to be changed.

Reviewed-on: https://git.koesters.xyz/timo/conduit/pulls/190


Reviewed-by: Timo Kösters <timo@koesters.xyz>