Implement separate connection limits per user

[justinbot]

Implements separate connection limits per user, to avoid scenarios where high traffic for a particular user can exhaust the connection pool and cause issues for other users.

This is configured by a new option homeserver.connection_limit_per_user (defaults to 10).
It can be used alongside homeserver.connection_limit (from #64) which still applies to the appservice itself.

Example addition to a config file:

homeserver:
    ...
    # Maximum number of simultaneous HTTP connections to the homeserver for a particular user.
    connection_limit_per_user: 5

[tulir]

This looks like a per-ghost connection limit, which I think in practice means there's just no limit? (because the bridge probably won't do that much stuff simultaneously with a single ghost user)