systemc: Store timed notifications in a list instead of a set.

This has three advantages. First, the data structure doesn't have to
try to keep track of whether or not an event is already listed there.
Second, it's easier to delete an item by storing an iterator for it
when it gets inserted. Third, the ordering of events is not dependent
on the arbitrary ordering of the set, it's bsaed on the fixed order
the events get added to the list.

One part of this change makes ScEvent-s keep track of what list they're
on, and handle their own insertion and deletion when they're
scheduled or descheduled. A side effect of that is that it's no longer
safe to simply use a range based for loop to loop over all of an
ScEvent and deschedule all its events or to run then (which deschedules
them internally once they execute).

That can be avoided by looping until the list is empty, and operating
on the first element. As the first element is processed and removed
from the list, the next element will become first and will get picked
up in the next iteration.

Change-Id: Icad51a63f153297c88e65f85d22ac721e6c571d8
Reviewed-on: https://gem5-review.googlesource.com/c/12456
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Improve scheduler cleanup.

Make the scheduler clear itself out when it's destructed to ensure that
nobody will try to use it after it's gone away. Also make sure there
are no pending events which might refer to it as well, either systemc
events or gem5 events.

Change-Id: I12dadc06bd9db7016a8dc0c1827b3e630b0d23d5
Reviewed-on: https://gem5-review.googlesource.com/12222
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Rework how delta and timed notifications/timeouts are tracked.

Rather than delegating them entirely to the gem5 event queue and using
priorities to ensure the right thing happens, this change adds a few
new structures which keep track of them and give the scheduler more
control over what happens and in what order. The old scheme was mostly
correct, but there were some competing situations which made it next
to impossible to make everything happen at the right time.

Change-Id: I43f4dd6ddfa488a31073c0318bb41369b1a6117d
Reviewed-on: https://gem5-review.googlesource.com/12213
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>