Fix for #501 and #479 (issues related to pausing a thread with debugger.set_suspend)

[fabioz]

This patch makes sure that:

• When a thread is suspended the stepping function is properly set (otherwise it may end up not breaking if the function was previously cached to be skipped).
• pydevd threads are not traced even if the tracing function is set on them.
• Changes settrace to let users pass where exactly the tracing should pause and use that API to make the pause from ptvsd as the previous approach had some issues if a given function had been previously skipped and currently had no tracing set.

[codecov-io]

Codecov Report

Merging #508 into master will decrease coverage by 0.07%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #508      +/-   ##
==========================================
- Coverage   60.16%   60.09%   -0.08%     
==========================================
  Files          27       27              
  Lines        3673     3666       -7     
==========================================
- Hits         2210     2203       -7     
  Misses       1463     1463

Impacted Files	Coverage Δ
ptvsd/attach_server.py	52.17% <0%> (+5.5%)	⬆️
ptvsd/wrapper.py	52.37% <0%> (-0.39%)	⬇️

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[DonJayamanne]

Why is trace_only_current_thread=True
Why can't and shouldn't we be tracing other threads?

[fabioz]

break_into_debugger will only make the pause for the current thread.

The method enable_attach is the one responsible for setting the tracing.

-- note: currently, because of the way that Python works, it will only set the tracing for future threads -- there's no API to set the tracing for a running thread from a different thread... not that it's impossible to do: the attach to running process has some utilities to do it, but it's currently not supported in the used APIs and that part from the attach to process needs a better API to handle this).