Keep the numbers heard as a list: a save logs only the ones it changed

The last number heard from each source was written whole into the links
value at every save, and counted in two bytes: a node that heard from
more than 65,535 sources stored a wrong count, and each save cost as many
bytes as there were sources. The numbers are now a runtime-owned list
{source, seq} in order of source, stored as a list state is (its image
"0heard" and its changes in the log), changed with the event that
delivers, so a failed save takes them back with it. Links stored before
(PLK1) move their numbers to the list at a start and are stored again
without them (PLK2).

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