High verbosity mode that prints arguments, hash and store location.

[japhba]

I found this feature useful when inspecting the synchronisation of caches between a remote and local machine. In particular, it can happen that arguments are hashed differently depending on the platform.
For example, this inspection revealed that

1. class-valued arguments were stored at a different location in memory and therefore had different hashes, as their str representation is different, and
2. some numerical arguments that did result from a numerical did not agree because of floating point inaccuracies in their value.

In logging the arguments and their hash, I was able to track down and resolve those differences so that syncing the caches does now work.

[codecov]

Codecov Report

Patch coverage: 96.00% and project coverage change: -0.15 ⚠️

Comparison is base (a007d79) 94.87% compared to head (5f6209b) 94.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1416      +/-   ##
==========================================
- Coverage   94.87%   94.72%   -0.15%     
==========================================
  Files          46       45       -1     
  Lines        6901     6918      +17     
==========================================
+ Hits         6547     6553       +6     
- Misses        354      365      +11     

Impacted Files	Coverage Δ
joblib/test/test_logger.py	92.85% <ø> (ø)
joblib/test/test_memory.py	98.47% <93.33%> (-0.11%)	⬇️
joblib/logger.py	87.17% <100.00%> (+0.33%)	⬆️
joblib/memory.py	95.36% <100.00%> (+0.09%)	⬆️

... and 12 files with indirect coverage changes

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[GaelVaroquaux]

Overall the changes seem acceptable and useful. Thanks a lot.

Could you add a test that does some rough check that the added functionality works right. If we don't do this, the danger is that later changes break the functionality.

Cheers

[tomMoral]

A couple of comments but I agree this @GaelVaroquaux , this functionality is nice and is something that I have been thinking of for quite some time.
Thanks for the PR and for adding tests.

A couple of comments along the way on style.

[tomMoral]

I am unsure whether this test has border effects, as the logging API relies on global handlers.
Would it be possible to create a local handler that we remove at the end of the test?

That being said, not completely sure this test is needed, maybe we could just remove it as this tests logger functionalities that are already tested by Python?

[tomMoral]

LGTM, thx @japhba

The failure on pypy is unrelated, so merging this one.