Refactor the Engine to process metrics independently from the test Run

[na--]

This allows metric processing to still occur, even after the script run has completed. In practice, this allows you to run k6 with the --linger --no-teardown flags, and execute the teardown() function manually through the REST API, after the main test run has completed.

This also fixes #1369 - now a second Ctrl+C will always abort k6, albeit in a very abrupt way. On the other hand, the first Ctrl+C would be more graceful than it currently is, since it will now allow teardown() to be executed.

[codecov-io]

Codecov Report

Merging #1390 into new-schedulers will decrease coverage by 0.03%.
The diff coverage is 52.17%.

@@                Coverage Diff                 @@
##           new-schedulers    #1390      +/-   ##
==================================================
- Coverage           76.39%   76.36%   -0.04%     
==================================================
  Files                 162      162              
  Lines               12838    12853      +15     
==================================================
+ Hits                 9808     9815       +7     
- Misses               2513     2518       +5     
- Partials              517      520       +3     

Impacted Files	Coverage Δ
cmd/run.go	9.28% <0.00%> (-0.16%)	⬇️
lib/execution.go	91.79% <ø> (ø)
core/engine.go	88.03% <85.71%> (-3.42%)	⬇️
core/local/local.go	70.30% <85.71%> (+0.13%)	⬆️
stats/cloud/collector.go	78.70% <0.00%> (+0.61%)	⬆️
lib/helpers.go	100.00% <0.00%> (+4.16%)	⬆️

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[mstoykov]

Seems fine to me :).

A bit more code then I hoped for, but looks okay to me. Will probably need to run a lot of tests, but if the automatic ones pass and you say it doesn't die running I am fine with a merge.

All my comments are about things that have little to do with this change and we can do it in another PR

[imiric]

No major comments from my side either. Not a big fan of the lambdas returned from Engine.Init(), it feels like this should be better integrated somehow. The new contexts complicate things a bit, but it also makes execution clearer, and it was necessary to properly handle linger, so it's OK.