This enables the consumer to override Date/performance object usage once
at the point of LRU object creation.

This is an affordance to make the TTL behavior easier to test in some
scenarios, without sacrificing security or performance. It should most
likely not be used in production!

Re: isaacs#345

PR-URL: isaacs#386
Credit: @isaacs
Close: isaacs#386
Reviewed-by: @isaacs

Fix: isaacs#387

Very niche edge case, but happens when a fetch implementation is
aborted, but the underlying fetch implementation was not aborted, and no
value existed in the cache already.

In this case, it *should* update the stored cached value, but was not,
because the pending fetch was not stored.

Fix: isaacs#388

This prevents a possible runaway resource utilization issue when using
ttlAutopurge.

If a single key is written to repetedly, then many NodeJS.Timer objects
will be created (or equivalent in the browser), and they are not
unscheduled when no longer needed. This results in many checks for
staleness happening unnecessarily, and so on.

Now, when a key is overwritten, set with no TTL, or any other behavior
that will mean that autopurge is no longer necessary, the timer is
cleared and deleted.