Rewrite promise implementation

[jgraichen]

This is a large rewrite of the promise implementation, replacing the comparable slow Concurrent::IVar with a lightweight custom implementation and specifying a few edge cases.

1. IVar used several Mutex and events internally and synchronized more than necessary. The new implementation uses a single Mutex, reducing contention and improving performance.

2. Do not reject a promise on a timeout. Waiting on a promise with a timeout that expires will not change the promise's state, allowing subsequent waits to continue where the last one left off. Because the previous simple implementation was driven by exceptions, every exception, including a timeout, rejected the promise. Now, it is possible to wait with different timeouts.

3. Honor the timeout of all threads waiting on the same promise, not only of the thread running its #then blocks.

   Before, all waiting happened on the Mutex synchronization, but that does not have a timeout at all. A long-running then-block being run on one thread could block other threads waiting on the same promise even if they had short timeouts.

   The new implementation separates between running and claiming/waiting to run, so threads with short timeouts are not blocked. Furthermore, because a timeout no longer affects the state, longer timeouts can still drive a promise to completion.

Technically, the new promise implementation is no longer based on Concurrent::IVar and does not inherit from it anymore. A promise could have been used in other concurrent-ruby contexts or any other Concurrent::IVar as a dependency. This is no longer possible!

Lately, these changes reduce contention and improve the overall performance quite a bit. Additionally, a lot fewer objects are allocated.

Which methods shall become public or not is still under consideration.

[codecov]

Codecov Report

❌ Patch coverage is 99.07407% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.35%. Comparing base (c5e11ee) to head (cfeecdf).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/restify/promise.rb	98.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
+ Coverage   97.23%   97.35%   +0.11%     
==========================================
  Files          28       28              
  Lines        1121     1171      +50     
==========================================
+ Hits         1090     1140      +50     
  Misses         31       31              

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