Reorder callbacks by their latest executable time

[LiyouZhou]

i.e. by scheduled_time + tolerance

[bogdanm]

As explained before, I'm hesitant to merge this kind of change before the release, since it might break people's code (see the issue you're working on with Marcelo) and it would require a new major version. It feels too hasty before an official release.
Cc @bremoran though.

[LiyouZhou]

@bogdanm I'm ok with postponing merge. I just want this to be on record for discussions.

[bogdanm]

That's perfectly fine then, thanks.

[bremoran]

Maybe I don't understand what this PR is supposed to do. My understanding is that call_before is already supposed to include the tolerance of the callback. This looks like we are now including double the tolerance: https://github.com/ARMmbed/minar/pull/34/files#diff-7b622de8de42474bad104c534c61d2c2R260

I'd like to see some measurements to demonstrate that this is doing the right thing.

[LiyouZhou]

call_before does not contain the tolerance:
https://github.com/LiyouZhou/minar/blob/reorder_by_late_exe_t/source/minar.cpp#L351
https://github.com/LiyouZhou/minar/blob/reorder_by_late_exe_t/source/minar.cpp#L386

[LiyouZhou]

A simple test was written to demonstrate and test the functionality. https://github.com/ARMmbed/minar-tests/blob/reordering_tests/test/MINAR037_tolerance_based_reordering.cpp