[DUX-1656] Multiple of the same lifecycle hook can run at once

[9999years]

This causes issues with e.g. hiedb (see https://github.com/wz1000/HieDb/issues/63).

Async hooks can run multiple instances at the same time, if the hook is slow enough and the reloads are fast enough. We should either allow only one of each hook to run at once or kill old hook processes before launching new ones. This should probably also be fixed upstream in hiedb.

_{From SyncLinear.com | DUX-1656}