Tests (external?) for verifying and testing pool sizing behavior under load

[dmlloyd]

Opened at the request of @franz1981.

We could use some testing to show how the EQE pool size expands and contracts under different kinds of load. The ability for the pool to grow and shrink while under load is one of the key features of EQE and it would be good to examine this behavior in detail.

Specifically, we would expect that the pool would shrink if there is not enough concurrent work to occupy all threads. We observe thread timeout behavior in the test suite, however testing under load is more challenging and is not usually practical or feasible as a unit test.