[status-] decrease latency of status() when other threads run

[midichef]

While benchmarking, I noticed that vd.status() is slow to finish when vd is running other very busy threads (taking ~150ms). If there are no busy threads, it's much faster, (taking ~2ms).

By recording the run time in memory, I found that it's only a matter of latency. That is, inspect.stack() takes a lot of clock time to finish, around 150 milliseconds, but very little of that time is spent in the thread. I don't know why, but I assume stack() pauses to let the busy threads run.

It turns out inspect.stack() is slow because it reads source files. And it can be made much faster by not asking for lines of source file context.

Here is a comparison of the latency, before and after this PR.

Before:
Using inspect.stack(), while loading a 2 million line tsv file takes 150-190ms per call to status():
quit.vdj.txt

seq 2000111 > 2m.tsv
# load 2 million line tsv file then quit
vd -p quit.vdj
status() time spent in inspect.stack(), total time:
mean clock time:      0.152629 seconds
status() time spent in inspect.stack(), in thread:
mean time in thread:  0.002206 seconds

After:
using inspect.stack(0), it takes only 40-50ms:

vd -p quit.vdj
status() time spent in inspect.stack(), total time:
mean clock time:      0.051474 seconds
status() time spent in inspect.stack(), in thread:
mean time in thread:  0.001276 seconds

[saulpw]

This makes sense, thanks @midichef !

[anjakefala]

Wow, the speed increase is incredible.

[ajkerrigan]

Chiming in late here just to say thanks for making and explaining this change. As a drive-by observer this was a fun TIL 👏