kvs: support configuration of max transaction count

[chu11]

Per discussion in #6125, denial-of-service attacks could be made against the KVS by very very large KVS transactions.

Support two configurations for capping the number of transactions made by users. One for each individual transaction made by a caller and one for the combined total of operations from a fence.

For the time being, I made the default 64K for the transaction cap and 1M for the fence cap.

I made this WIP only b/c those defaults may be tweaked depending on what stats we get from the prior PR #6556. I would like to merge only after we gather a bit of data, although I'd be quite shocked if we have to adjust the defaults. Edit: Or alternately, if we'd like to just get the code in, we could default the max to LLONG_MAX and lower the default at a later time.

Only other thought is I decided to return the errno E2BIG if we went across a max cap boundary. It's possible there is a superior errno for this, I picked it b/c I thought "ehhh that's not bad".

[chu11]

re-pushed, removing all fence related parts of this PR, since KVS fence support is now gone

[chu11]

re-pushed, removing all the fence stuff that is no longer relevant b/c KVS fence support was removed

[chu11]

Some small set of transaction stats

 "transaction-opcount": {
  "commit": {
   "count": 48829,
   "min": 1,
   "mean": 1.976673698007309,
   "stddev": 6.318251377545355,
   "max": 398
  }
 },

the max number of ops in a transaction is 398, which means my default cap of 64K seems more than fine. I'll still monitor stats until release time, but removing WIP for now so this can be reviewed

[chu11]

saw this today

 "transaction-opcount": {
  "commit": {
   "count": 95589,
   "min": 1,
   "mean": 3.4195984893659341,
   "stddev": 42.249674028063289,
   "max": 11395
  }
 },

max of 11K ... perhaps upping to 128K over 64K would still be good but a bit safer.

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.86%. Comparing base (c57d3b1) to head (a8e4ffa).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/modules/kvs/kvs.c	90.90%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6581   +/-   ##
=======================================
  Coverage   83.86%   83.86%           
=======================================
  Files         533      533           
  Lines       88683    88705   +22     
=======================================
+ Hits        74375    74394   +19     
- Misses      14308    14311    +3     

Files with missing lines	Coverage Δ
src/modules/kvs/kvs.c	74.58% <90.90%> (+0.26%)	⬆️

... and 6 files with indirect coverage changes