Changing sync interval adds new periodic workers, not updating or removing the existing one

[sunkup]

Changing sync interval adds new periodic workers, not updating or removing the existing one.

Reproduce:

1. Add subscription
2. change the sync interval
3. See the PeriodicSyncWorkers pile up in the Background Task Inspector

Might be due to 2c05b63 , but I don't think so, since ExistingPeriodicWorkPolicy#UPDATE should handle this accordingly?

One might think the old work is stopped or cancelled once it attempts to run, but that does not seem to be the case ... Only after setting it to manually once and then to some timer interval will the periodic sync workers get removed.

@ArnyminerZ Can you try reproducing this as well?

We might need to switch back #REPLACE or rather #CANCEL_AND_REENQUEUE since that seems to be the new name for REPLACE.