[#28142][Go SDK] Evict closed readers from the cache.

[lostluck]

Resolve a memory leak as a result of the Data Manager read cache not evicting data streams for which it had not known it had seen all the data streams.

See the new comment in the code for details.

We don't take the alternative approach of checking the DataChannel's endedInstruction cache because it's possible that cache of instruction references might have a gap, with more than the current cache cap of ended instructions cycling through before the data channel is triggered to observe it. Best to check the cache's contents directly.

Fixes #28142.

I'm not sure how to test for this unfortunately without significantly changing the code, as it's a goroutine local cache, and adding additional synchronization is undesirable.

While this regression has been in the SDK for a while (since around February 2023) I'd like to cherry pick it into the 2.54.0 release so the ever patient user can finally upgrade to a newer SDK version.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3ae8518) 38.42% compared to head (4a09c43) 38.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #30119   +/-   ##
=======================================
  Coverage   38.42%   38.42%           
=======================================
  Files         697      697           
  Lines      102214   102217    +3     
=======================================
+ Hits        39272    39275    +3     
  Misses      61315    61315           
  Partials     1627     1627           

Flag	Coverage Δ
go	54.18% <100.00%> (+<0.01%)	⬆️

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