Add additional locking in DemandBuffer to protect demandState

[simba909]

I've recently been building a feature where I'm setting up a publisher using Publishers.Create on the main thread, and then sending values to the generated subscriber from another thread (this might an unsupported use-case for this publisher, let me know if so!). When debugging the code with Thread Sanitizer I get a warning about an access race:

SUMMARY: ThreadSanitizer: Swift access race (MyProject:arm64+0x1014d7318) in DemandBuffer.buffer(value:)+0x7c0

Looking at the code I can see that a NSRecursiveLock is used to protect the demandState inside of the DemandBuffer, but only in the flush() function. Since the demandState property is also accessed from within the buffer(value:) function my hunch is that this too should be protected by the lock. The use of the lock here leads me to believe that my threading case might be valid, but again let me know if this isn't the case.

This PR adds locking to the buffer(value:) function. Tests all seem to pass, and Thread Sanitizer is no longer upset with my code 👍🏻

[freak4pc]

Looks like a good fix to me. Thanks for catching it!
Can you fix the one minor formatting thing here?

[simba909]

@freak4pc Thank you for the review! Committed your suggested formatting change 👍🏻

[codecov]

Codecov Report

Merging #90 (60104fa) into main (730d272) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 60104fa differs from pull request most recent head 25958f0. Consider uploading reports for the commit 25958f0 to get more accurate results

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   97.12%   97.06%   -0.06%     
==========================================
  Files          62       62              
  Lines        3336     3338       +2     
==========================================
  Hits         3240     3240              
- Misses         96       98       +2     

Impacted Files	Coverage Δ
Sources/Common/DemandBuffer.swift	100.00% <100.00%> (ø)
Sources/Operators/Internal/Timer.swift	67.76% <0.00%> (-1.32%)	⬇️

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