Grudge keeps a high number of active blocks

When I run the wave-eager case with a memory pool, I observe the number of active blocks in the memory pool can reach almost 250. Nearly every call to `actx.empty` or `actx.from_numpy` adds another active block. Assuming `pn=3`, for nel_1d=32 (~173,000 elements) and below there is sufficient device memory to execute. For nel_1d=64 (1.5 million elements) the allocations are ~0.25 GB and all of device memory is eventually consumed.

I presume there are a lot of old allocations hanging around that ought to be freed. Perhaps this is related to https://github.com/inducer/pyopencl/issues/450.