WIP kvs-watch: do not load all KVS blobs at once

[chu11]

Problem: With the FLUX_KVS_WATCH_APPEND and FLUX_KVS_STREAM flags, all content blobs in a valref treeobj will be retrieved from the content store at once. If the valref array is gigantic, this may be a very costly initial transaction. Not to mention, we probably wouldn't want to send an extremely large number (like millions) of content requests all at once.

Solution: Send content requests in more reasonable 32K chunks.

Fixes #6456

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So this is what I initially started working on that lead more to #6531 and #6518.

This issue is mostly theoretical, as I don't have a use case where this is a problem at the moment. I want to try and manufacture a use case and test it, which is why I'm posting this as WIP. But I thought I'd post and see if folks think it's atleast a good idea.

I should note, I did run the test suite when having the "max loads" set to 2 instead of 32K, so it generally speaking appears to work safely. Although the question of how big the "maximum number of loads at a time" number could possibly be tweaked up or down some.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 8 lines in your changes missing coverage. Please review.

Project coverage is 83.65%. Comparing base (9278fa8) to head (8604a3d).
Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
src/modules/kvs-watch/kvs-watch.c	83.33%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6532      +/-   ##
==========================================
+ Coverage   83.62%   83.65%   +0.02%     
==========================================
  Files         522      522              
  Lines       87809    87829      +20     
==========================================
+ Hits        73433    73473      +40     
+ Misses      14376    14356      -20     

Files with missing lines	Coverage Δ
src/modules/kvs-watch/kvs-watch.c	77.93% <83.33%> (-0.38%)	⬇️

... and 8 files with indirect coverage changes