Make OptionTypeConverter thread-safe

[cmeeren]

Follow-up of #6883.

I realized that using Dictionary with double-checked locking means that it's possible one thread calls TryGetValue while another thread modifies the dictionary, which AFAIK is not allowed for Dictionary. This PR uses a ConcurrentDictionary instead, which avoids that issue.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c5a043e) 73.29% compared to head (00fab63) 73.32%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6891      +/-   ##
==========================================
+ Coverage   73.29%   73.32%   +0.03%     
==========================================
  Files        2584     2584              
  Lines      128343   128343              
==========================================
+ Hits        94069    94109      +40     
+ Misses      34274    34234      -40     

Flag	Coverage Δ
unittests	73.32% <ø> (+0.03%)	⬆️

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[michaelstaib]

OK.. I did not review the F# PRs so far ...

You would not use a lock in combination with a ConcurrentDictionary.
GetOrAdd(TKey, TValue)

The locking implemented with the ConcurrentDictionary is far more efficient as the lock on the buckets in the dictionary.

[cmeeren]

ConcurrentDictionary guarantees "only" consistency during reads/updates. The locking prevents doing the work of generating the value for a key more than once in case there are multiple threads trying to generate the value for the same key at the same time.

However, in this case, that seems like an unnecessary optimization, since the difference between doing the work once and doing it a few times is unlikely to matter.

I will update the PR.

[cmeeren]

Done.