The message stores don't need to access the internal network
struct, they just need network metadata such as ID and name.
This can ease moving message stores into a separate package in the
future.
This allows to have shorter and more future-proof IDs. This also
guarantees the IDs will only use reasonable ASCII characters (no
spaces), removing the need to encode them for PING/PONG tokens.
Introduce a messageStore type, which will allow for multiple
implementations (e.g. in the DB or in-memory instead of on-disk).
The message store is per-user so that we don't need to deal with locking
and it's easier to implement per-user limits.
