* Avoid circular import in Storage.async_delay_save
We call Storage.async_delay_save for every entity being added or removed
from the registry. The late import took more time than everything else
in the function.
* Avoid reschedule churn in Storage.async_delay_save
When we are adding or removing entities we will call async_delay_save
quite often which has to add and remove a TimerHandle on the event loop
which can add up when there are a lot of registry items changing.
If the timer handle still has 80% of the time remaining on it
we will avoid resceduling and let it fire at the time the
original async_delay_save call was made. This ensures we
do not force the event loop to rebuild its heapq because
too many timer handlers were cancelled at once
* div0
* add coverage for 0 since we had none
* fix bad conflict
* tweaks
* tweaks
* tweaks
* tweaks
* tweaks
* tweaks
* more test fixes
* mqtt tests rely on event loop overhead
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