cache: throttle wakeups

In the extreme case, reading 1 byte would wake up the cache to make the
cache thread read 1 byte. This would be extremely inefficient. This will
not normally happen in our cache implementation, but it's still present
to some lesser degree. Normally you'd set a predefined "cache too low"
boundary, after which you would restart reading. For some reason
something like this is already present using a hardcoded value
(FILL_LIMIT - I don't even know the deeper reason why this exists). So
use that to reduce wakeups.

This doesn't fix redundant wakeups on EOFs, which is especially visible
should something keep retrying reading on EOF (like in an endless loop).

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