Add diagnostic method `pooledCount()` in `RecyclerPool`

[cowtowncoder]

Currently there is no way to get an estimate number of recyclable entries in a RecyclerPool.
This is fine for regular use, but for operational reasons, and (more importantly), testing purposes it would be good to have a way to get even just an estimation.
So let's add something like RecyclerPool.pooledCount() to support this.

[cowtowncoder]

Also backport in 2.17(.1) for testing purposes although officially only part of 2.18 API.