kvs: return ENOSYS on unfinished requests

[chu11]

Problem: When the KVS module is unloaded, any unfinished requests (get, put, wait-version, etc.) should get an ENOSYS response. That is presently not handled.

Solution: Track all requests sent to the kvs module that will not be returned immediately. Upon module exit, respond to all unfinished requests with ENOSYS.

Fixes #5979

---

note, I noted in #5979 that some other modules might need this fix as well. Could tack onto this PR as well, but thought we'd deal with just this one first since it specifically has been hit.

[chu11]

rebased & re-pushed, removing the move of msg_hash from librouter to libfluxutil.

[chu11]

re-pushed, fixing up things per comments above and adding some extra tests. The only real "gotcha" on the tests is that some "pending_requests" exist in one test at the end, so those are manually counted. This is related to the recently opened #6112

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.75000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.36%. Comparing base (de5d907) to head (176eea4). Report is 531 commits behind head on master.

Files with missing lines	Patch %	Lines
src/modules/kvs/kvs.c	93.75%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6049      +/-   ##
==========================================
- Coverage   83.37%   83.36%   -0.01%     
==========================================
  Files         521      521              
  Lines       84681    84719      +38     
==========================================
+ Hits        70601    70626      +25     
- Misses      14080    14093      +13     

Files with missing lines	Coverage Δ
src/modules/kvs/kvs.c	71.87% <93.75%> (+0.51%)	:arrow_up:

... and 10 files with indirect coverage changes