Use PurpleProtocol.can_connect_async in the login process

Also implement it for all the new protocol plugins.

Testing Done:
Attempted connections for the new bonjour, ircv3, and new xmpp accounts and verified them made it past the checks.
Attempted and ircv3 account with a bad DNS name and verified it failed in the `can_reach_async` functions.
Also ran with the turtles.

Reviewed at https://reviews.imfreedom.org/r/3005/