firebase-ios-sdk

Don't use the address of a TimeSlot as a key for removal (#4212)

After a TimeSlot executes, it deletes itself and removes itself from
ExecutorLibdispatch's schedule_. The DelayedOperation still refered to
this now freed address. If another TimeSlot is scheduled and operator
new happens to allocate a TimeSlot that reuses the address, and the
DelayedOperation is subsequently canceled, this will cancel the new task
before it has run.

This change fixes this by using a dedicated counter for TimeSlots that
effectively isn't reused. Note that while it is possible for the counter
to wrap around, this would take tens of years of continuous execution to
happen and require cancelation of TimeSlot created tens of years ago to
create the possibility of collision which is a sufficiently impractical
problem as to be ignorable.