Pool.php - add methods to clear finished/results cache (#235)

[iasjennen]

Since the member variables of finished/results keep growing indefinitely over time this is a source of a memory leak. This memory leak is resolvable by clearing the result caches with the newly introduced methods.

[Nielsvanpach]

Could you add tests to ensure this works?

[iasjennen]

@Nielsvanpach Please find the test now attached.
This test also checks for memory consumption before and after result clearing. Please note the memory_get_usage() may be a bit fuzzy to interpret and i am not sure whether the pool is still memory leaking at all after the clearing.
Anyway the saving seems to be at least in the range of an order of magnitude.