xdsclient: stop batching writes on the ADS stream

[easwars]

Fixes #8125

The original race in the xDS client:

• Resource watch is cancelled by the user of the xdsClient (e.g. xdsResolver)
• xdsClient removes the resource from its cache and queues an unsubscribe request to the ADS stream.
• A watch for the same resource is registered immediately, and the xdsClient instructs the ADS stream to subscribe (as it's not in cache).
• The ADS stream sends a redundant request (same resources, version, nonce) which the management server ignores.
• The new resource watch sees a "resource-not-found" error once the watch timer fires.

The original fix was to delay the resource's removal from the cache until the unsubscribe request was transmitted over the wire, a change implemented in #8369. However, this solution introduced new complications:

• The resource's removal from the xdsClient's cache became an asynchronous operation, occurring while the unsubscribe request was being sent.
• This asynchronous behavior meant the state maintained within the ADS stream could still diverge from the cache's state.
• A critical section was absent between the ADS stream's message transmission logic and the xdsClient's cache access, which is performed during subscription/unsubscription by its users.

This PR simplifies the implementation of the ADS stream by removing two pieces of functionality

• Stop batching of writes on the ADS stream
  • If the user registers multiple watches, e.g. resource A, B, and C, the stream would now send three requests: [A], [A B], [A B C].
• Don't buffer writes when waiting for flow control
  • Flow control is already blocking reads from the stream. Blocking writes as well during this period might provide some additional flow control, but not much, and removing this logic simplifies the stream implementation quite a bit.

RELEASE NOTES:

• xdsclient: fix a race in the xdsClient that could lead to resource-not-found errors

[codecov]

Codecov Report

❌ Patch coverage is 79.41176% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.10%. Comparing base (8389ddb) to head (54c1664).

Files with missing lines	Patch %	Lines
internal/xds/clients/xdsclient/ads_stream.go	81.81%	2 Missing and 2 partials ⚠️
internal/xds/clients/internal/buffer/unbounded.go	75.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8627      +/-   ##
==========================================
- Coverage   82.13%   82.10%   -0.03%     
==========================================
  Files         415      415              
  Lines       40711    40677      -34     
==========================================
- Hits        33437    33397      -40     
- Misses       5897     5906       +9     
+ Partials     1377     1374       -3     

Files with missing lines	Coverage Δ
internal/xds/clients/internal/buffer/unbounded.go	94.23% <75.00%> (-5.77%)	⬇️
internal/xds/clients/xdsclient/ads_stream.go	82.83% <81.81%> (-1.32%)	⬇️

... and 26 files with indirect coverage changes

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[easwars]

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