Avoid a KeyError when a ComponentLocator is being called concurrently

[akx]

The symptom is e.g. KeyError: 'credential_provider', detailed in e.g. #1246 (comment)

As detailed in the comment, even if the component factory gets called twice, cleaning the deferred up should not cause a KeyError.

[nateprewitt]

Hi @akx, thanks for the PR. This seems like a reasonable thing to fix, but we should caveat that this case should never be hit under documented usage. Sessions are not thread safe and client creation shouldn't be done in parallel using the same session. We have the details on usage of Sessions around parallelization primitives here.

[akx]

@nateprewitt I kind of understand that, but it really isn't very obvious that you should not use boto3.client() (that uses the implicit shared default session) from more than one thread.

Maybe the default session should be thread-local or plain refuse to work between threads if it's not actually supported?

[codecov-commenter]

Codecov Report

Patch coverage: 50.00% and no project coverage change.

Comparison is base (59edb73) 93.36% compared to head (bcb1f8e) 93.37%.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2985   +/-   ##
========================================
  Coverage    93.36%   93.37%           
========================================
  Files           65       65           
  Lines        13841    13844    +3     
========================================
+ Hits         12923    12927    +4     
+ Misses         918      917    -1     

Impacted Files	Coverage Δ
botocore/session.py	97.20% <50.00%> (-0.45%)	⬇️

... and 1 file with indirect coverage changes

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[nateprewitt]

I think we cover this to some extent in the Session section and we note that boto3.client uses the default session in the docstring, but I agree it could be easier to find. I'll open an issue for that and merge this to help in the cases this is missed.

[nateprewitt]

#2991 created for reference.