Add profiling to benchmarks/tpch. Improve profiler to prevent VizTracer nesting.

[xcharleslin]

Add daft.runners.profiler to benchmarks.tpch. Now, you can run benchmark with DAFT_PROFILING=1 to generate a profile per question.

The profile will be in the form tpch_q{num}_{timestamp}.json.

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Adding the profiler to TPCH introduces nested profiling, since there are already profiler calls lower in the Daft code. However, VizTracer does not seem to be nestable. It will complain loudly:

Warning! Overwrite tracer! You should not have two VizTracer recording at the same time!

and then sometimes segfault:

*** SIGSEGV received at time=1672950845 on cpu 2 ***
PC: @     0x7fc5f9ea6306  (unknown)  (unknown)
    @     0x7fc5f9c12320  (unknown)  (unknown)
[2023-01-05 12:34:05,190 E 2576853 2577214] logging.cc:361: *** SIGSEGV received at time=1672950845 on cpu 2 ***
[2023-01-05 12:34:05,190 E 2576853 2577214] logging.cc:361: PC: @     0x7fc5f9ea6306  (unknown)  (unknown)
[2023-01-05 12:34:05,191 E 2576853 2577214] logging.cc:361:     @     0x7fc5f9c12320  (unknown)  (unknown)
Fatal Python error: Segmentation fault

Stack (most recent call first):
<no Python frame>
zsh: segmentation fault (core dumped)

To work around this, I made Daft's viztracer wrapper its own context manager that no-ops if an existing VizTracer is already active.