cmd/syncthing: Send Ping events to the disk events API
cmd/syncthing: Send Ping events to the disk events API

The Ping event is important, as it means that requests complete within
a sensible time. The disk events API didn't have the Ping event, so
if there were no disk events, the request would keep taking forever.
Unless, of course, there's a reverse proxy which times the request out
after a suitably large interval (or something else aborts it), in which
case Syncthing isn't very happy.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3929

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