kvs-watch: Support FLUX_KVS_WATCH_UNIQ

[chu11]

Per #1847. The addition was even more trivial than I had originally suspected.

[codecov-io]

Codecov Report

Merging #1850 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1850      +/-   ##
==========================================
+ Coverage   79.89%   79.92%   +0.02%     
==========================================
  Files         196      196              
  Lines       35300    35302       +2     
==========================================
+ Hits        28204    28216      +12     
+ Misses       7096     7086      -10

Impacted Files	Coverage Δ
src/modules/kvs-watch/kvs-watch.c	76.99% <100%> (+0.76%)	⬆️
src/cmd/flux-kvs.c	83.52% <100%> (+0.03%)	⬆️
src/common/libkvs/kvs_lookup.c	81.87% <100%> (-0.11%)	⬇️
src/broker/modservice.c	78.84% <0%> (-0.97%)	⬇️
src/modules/kvs/kvs.c	65.65% <0%> (-0.16%)	⬇️
src/common/libflux/message.c	81.51% <0%> (-0.13%)	⬇️
src/cmd/flux-event.c	77.97% <0%> (ø)	⬆️
src/modules/connector-local/local.c	74.81% <0%> (+1.18%)	⬆️
src/modules/barrier/barrier.c	78.62% <0%> (+2.06%)	⬆️

[garlick]

Nice that this is not a lot of code! LGTM.

[chu11]

let me do @grondo's little fix and I'll re-push

[garlick]

Thanks!