Reject conficting challenge registrations

[poszu]

Closes #340

[codecov]

Codecov Report

Merging #341 (e2371fa) into develop (beb3bdf) will decrease coverage by 0.1%.
The diff coverage is 75.0%.

@@            Coverage Diff            @@
##           develop    #341     +/-   ##
=========================================
- Coverage     73.1%   73.1%   -0.1%     
=========================================
  Files           17      17             
  Lines         1561    1568      +7     
=========================================
+ Hits          1142    1147      +5     
- Misses         315     318      +3     
+ Partials       104     103      -1     

Files Changed	Coverage Δ
service/service.go	76.2% <ø> (ø)
service/round.go	78.4% <71.4%> (-0.4%)	⬇️
rpc/rpcserver.go	78.8% <100.0%> (+0.5%)	⬆️