Shrink the size of memory pools when usage decreases

[halter73]

Currently the memory pool implementation will only grow in size. This means if there is burst of high load that subsided, memory usage for the Kestrel process will not decrease as much as it could.

[benaadams]

Apply common server/cluster load balancing model?

Pools per slab, round robin rents from available slabs till available block? Move slabs move out of rotation; but accept returns. When all blocks are returned dispose slab; if alloc is needed prior, move slab back into rotation; if no slabs being deallocated allocate new.

[benaadams]

Probably need some heuristics on whether growing, shrinking or static to determine whether to take slabs out of rotation

[muratg]

Closing because @davidfowl thinks this is not problem.