Remove (::ConnectionRule)(src, dst, t) fallback method; make discrete_event_condition on connections take src / dst args.

[MasonProtter]

This makes the following breaking API updates:

1. Removed the fallback (cr::ConnectionRule)(src, dst, t) = cr(src, dst) which was added previously to avoid breakage when we made connection rules take a time argument in addition to the src and dst subsystems. The presence of this method was a bit of a annoying crutch
2. Changed the arguments of discrete_event_condition from just (conn, t) to (conn, t, sys_src, sys_dst) so that events can trigger based off information in the source or destination subsystems as well
3. Changed the arguments of has_discrete_events from just (typeof(conn),) for connection events to (typeof(conn), get_tag(src), get_tag(dst)), i.e. it now also can depend on the types of the source and dest subsystems
4. Changed the arguments of event_times from just (conn,) to (conn, sys_src, sys_dst) for connection events.

cc @vyudu