- host_is_up returned the exception object for a down host, which is
truthy: check_user_quota then connected to it and failed, so a user
with a VM on a down host could not open its page, clone or resize,
and migrate_instance's "target is down" check never fired. It returns
True or False now, closes its socket, and accepts IPv6 and host:port.
- The connection manager no longer holds its global lock while a new
connection is opened, so one slow host does not delay new connections
to the others.
Bounding the connection time itself (including SSH aliases and jump
hosts) is left for a separate change.
