Allow invalidation of entries based upon a capacity target

[cetra3]

I'll try my best to explain the scenario.

We have a cache that stores files as Byte values and we set the weigher as Bytes::len() and the max capacity to some number of bytes (i.e, 1gb or something).

When populating the cache either with try_get_with or insert, or whatever, there is a point in time where new memory is allocated in either the init future, or equivalent.

So what ends up happening is that your total size in memory of the cache is roughly equal to:

max_capacity + memory of init futures

When those init futures resolve, an eviction happens, and memory goes down. But for a period of time, there can be a lot of overhead in those init futures.

What I think might work is that, since within the init futures we know what the size of the thing will be when it goes in the cache before allocating, if we could say to the cache something like "evict up to n amount of capacity", then it would allow us to keep memory under control.

Something like:

cache.invalidate(capacity_num)

Let me know if that doesn't make sense, or there are any alternatives

[cetra3]

Just to update this: I've managed to work around this by using a shared semaphore in the init futures. I still think it might be valuable to have a way to resize the cache dynamically, but I will close this ticket off.