resource: round timestamp of drained ranks

[grondo]

This PR rounds the drain timestamp in the resource module while building the drain object for a resource.status response. The idea here is that this will allow more compression in the drain object in the response and also result in less lines of output in flux resource drain (when sub-second precision in the output is used)

A kind of best case scenario is show by separately draining 16000 nodes, such that each rank has a different timestamp (but many are actually within the same second)

before

expecting success:
        flux dmesg -c >/dev/null &&
        time rpc "resource.status" >rstatus.json
        flux dmesg -HL


real    0m0.766s
user    0m0.193s
sys     0m0.040s
ok 7 - time resource.status rpc

expecting success:
        time flux python -m cProfile -o prof.out $flux_resource status

     STATE UP NNODES NODELIST
     avail  ✔      1 fake0
    avail*  ✗    383 fake[16001-16383]
  drained*  ✗  16000 fake[1-16000]

real    0m6.965s
user    0m5.753s
sys     0m0.092s

expecting success:
        flux dmesg -c >/dev/null &&
        time rpc "resource.status" >rstatus.json
        flux dmesg -HL


real    0m0.292s
user    0m0.205s
sys     0m0.031s
ok 7 - time resource.status rpc

expecting success:
        time flux python -m cProfile -o prof.out $flux_resource status

     STATE UP NNODES NODELIST
     avail  ✔      1 fake0
    avail*  ✗    383 fake[16001-16383]
  drained*  ✗  16000 fake[1-16000]

real    0m5.524s
user    0m4.784s
sys     0m0.065s

Note the RPC time was reduced from 0.766s to 0.292s -- probably from massively reducing the payload size.
There's only a slight difference in flux resource status because the command is now bound by performance of librlist operations (mostly rlist_diff).

[garlick]

LGTM!

[codecov]

Codecov Report

Merging #5866 (52e9d0d) into master (42fdaaa) will decrease coverage by 0.03%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5866      +/-   ##
==========================================
- Coverage   83.29%   83.27%   -0.03%     
==========================================
  Files         514      514              
  Lines       82758    82758              
==========================================
- Hits        68937    68920      -17     
- Misses      13821    13838      +17     

Files	Coverage Δ
src/modules/resource/drain.c	78.50% <ø> (ø)

... and 10 files with indirect coverage changes

[grondo]

Thanks! Setting MWP