flux-wreck cancel should send jobid as JSON number

[grondo]

Fix for encoding of jobid argument in sched.cancel command sent by flux wreck cancel as noted by @dongahn.

Also included is some removal of duplicated code in flux wreckrun

[coveralls]

Coverage decreased (-0.06%) to 78.796% when pulling 574a5d4 on grondo:wreck-cancel-fixup into 0448c22 on flux-framework:master.

[codecov-io]

Codecov Report

Merging #1367 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1367      +/-   ##
==========================================
- Coverage   78.54%   78.48%   -0.07%     
==========================================
  Files         162      162              
  Lines       29741    29741              
==========================================
- Hits        23361    23342      -19     
- Misses       6380     6399      +19

Impacted Files	Coverage Δ
src/modules/connector-local/local.c	72.95% <0%> (-1.44%)	⬇️
src/broker/module.c	83.79% <0%> (-1.4%)	⬇️
src/broker/content-cache.c	73.43% <0%> (-1.3%)	⬇️
src/common/libflux/rpc.c	93.38% <0%> (-0.83%)	⬇️
src/common/libutil/dirwalk.c	93.57% <0%> (-0.72%)	⬇️
src/common/libflux/message.c	81.01% <0%> (-0.36%)	⬇️
src/cmd/flux-module.c	85.06% <0%> (-0.31%)	⬇️
src/common/libflux/future.c	88.31% <0%> (ø)	⬆️
src/modules/kvs/kvs.c	65.21% <0%> (+0.16%)	⬆️
src/broker/overlay.c	74.45% <0%> (+0.31%)	⬆️
... and 2 more