MINOR: pools: compute an estimate of each pool's average needed objects

MINOR: pools: compute an estimate of each pool's average needed objects

This adds a sliding estimate of the pools' usage. The goal is to be able
to use this to start to more aggressively free memory instead of keeping
lots of unused objects in pools. The average is calculated as a sliding
average over the last 1024 consecutive measures of ->used during calls to
pool_free(), and is bumped up for 1/4 of its history from ->allocated when
allocation from the pool fails and results in a call to malloc().

The result is a floating value between ->used and ->allocated, that tries
to react fast to under-estimates that result in expensive malloc() but
still maintains itself well in case of stable usage, and progressively
goes down if usage shrinks over time.

This new metric is reported as "needed_avg" in "show pools".

Sadly due to yet another include dependency hell, we couldn't reuse the
functions from freq_ctr.h so they were temporarily duplicated into memory.h.