Recover an appservice if a successful ping occurs. (#18521)

Fixes https://github.com/element-hq/synapse/issues/14240

This scratches an itch that i've had for years. We regularly run into
the issue where (especially in development) appservices can go down for
a period and them come back up. The ping endpoint was introduced some
time ago which means Synapse can determine if an AS is up more or less
immediately, so we might as well use that to schedule transaction
redelivery.

I believe transaction scheduling logic is largely implementation
specific, so we should be in the clear to do this without any spec
changes.

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