Stress test the ChainWorkers cache

[jvff]

Motivation

The ChainWorkers cache should usually contain at least a few idle workers. However, it should still operate normally under heavy load (as in, many new attempts for chain workers that aren't already in the cache). There should be tests to reproduce this scenario.

Proposal

Write unit tests that simulate many requests for a number of chain workers larger than the cache size. A simulated ChainWorkerRequest::PretendToBeBusy can be added for testing purposes. The variant can include a field to specify the Duration of the request, and there could be tests for long delays and short delays.