Track pending pings to retry later

[fluffy-critter]

Save the scheduled and failed pings into an item which gets written to the cache after each ping has completed (after either removing or marking as failed, accordingly). OR these items into the scheduled list when the target list is computed.

This will help with the case that Pushl gets interrupted partway through (or the receiving endpoint happens to be down/erroring at the time), so pings aren’t lost forever.

[fluffy-critter]

Potentially easier solution: only cache successful pings in the first place