Refactor executor contexts?

[na--]

This originally started as a PR comment: #1863 (comment)

The context hierarchy is a bit unexpected, we have:

parentCtx -> maxDurationCtx -> regDurationCtx
                           \-> maxDurationCtxWithScenarioState -> vuCtx

So someone expecting to have the ScenarioState in regDurationCtx might be surprised. Something like this might be better:

parentCtx -> parentCtxWithScenarioState -> maxDurationCtx -> regDurationCtx
                                                         \-> vuCtx

Unfortunately, there are some implementation chicken-and-egg problems that were described in the PR thread 😞

Other somewhat related things:

• cancelling the VU context at the end of an iteration, which recently came up in this forum thread, but might be required for event loops (Global JS event loops in every VU #882)
• test aborting ( Add abortTest() helper function #1920)
• various other refactoring of the executors, especially their huge Run() methods and other optimizations and cleanups (Refactor the externally-controlled executor and add tests #1524, Refactor the module structure for the new executors #1302, Optimize ramping-vus #1499)