Shashlik out of time pileup mitigation

Multifit for ecal pulse shapes backport from 7_something, and PF pileup mitigation changes.  Includes #5845.

@bachtis, I had to add a change to class checksums
@lgray, @bendavid, FYI
Very strange.  I'll take out this change and queue it up in a separate PR so that it can be applied quickly if the IB fails.