We have an AbortController on the Rust side that we can use to abort a merge, but it's only triggered at shutdown. Additionally, it's not currently possible to interrupt fetching records or notifying observers, both of which can be pretty expensive (see bug 1561467, in particular, that notifying observers took 18 seconds in one case. I think our use of Async.jankYielder means that we might wait for longer periods).

A while ago, Mark suggested having a watchdog timer automatically abort merges that take longer than a few minutes. 5 feels like a good starting point.

This commit changes mozISyncedBookmarksMerger.merge to return a
cancelable operation, and adds an abort signal option to
SyncedBookmarksMirror.apply. Aborting the signal's
AbortController, or finalizing the mirror, interrupts the merge.

This commit introduces a new Watchdog class that signals an abort,
either after a delay or at shutdown, and wires up the buffered engine
to use it.

Depends on D41310

https://hg.mozilla.org/mozilla-central/rev/9f104a3a1304
https://hg.mozilla.org/mozilla-central/rev/595541e822af