Possibility to gracefully stop sitespeed task with correct report generation

[S0bakaRU]

Feature/improvement

It would be great to have a possibility to gracefully interrupt the execution of a multi-iteration test at an arbitrary time.
Or the possibility to run sitespeed iterations based on some predefined time duration, not iteration counter.
It is important that after a "stop signal" is received, the current iteration ends correctly and sitespeed calculates the final reports based on the completed iterations.

Typical possible scenario:

Run sitespeed with --browsertime.iterations = 1000 in parallel with some load tests with indefinite duration and need to interrupt sitespeed when load test will be finished with final report generation based on really passed iterations.

It would be great to have the possibility to initiate the mentioned "stopping" action in some common automation-friendly way: like sending a custom linux signal, or setting some control file or another "flag" (similar way as it is realised for dashbaoard.sitespeed.io).

[soulgalore]

This would be cool, but I think it's hard to implement? A terminate signal could send the correct messages on the queue (signal that everything is done) but we need to clean up/stop the browser. A first step would be to try to implement this in Browsertime.