Register challenges in batches

[poszu]

Closes #357

The submitted challenges are collected in a batch that is written to disk (with sync):

• when it reaches max batch size (default: 1000)
• periodically (default: 100ms since the batch was created).

Comparison of TestLoadSubmits before and after on an NVME drive with default settings:

submitting 10000 challenges took 1m3.840035945s

vs

submitting 10000 challenges took 1.397493686s

[codecov]

Codecov Report

Merging #358 (e0648df) into develop (41d1da3) will increase coverage by 1.9%.
The diff coverage is 94.1%.

@@            Coverage Diff            @@
##           develop    #358     +/-   ##
=========================================
+ Coverage     71.7%   73.6%   +1.9%     
=========================================
  Files           20      20             
  Lines         1749    1859    +110     
=========================================
+ Hits          1255    1370    +115     
+ Misses         366     363      -3     
+ Partials       128     126      -2     

Files Changed	Coverage Δ
service/service.go	76.7% <80.0%> (-0.2%)	⬇️
service/round.go	83.1% <98.8%> (+5.9%)	⬆️
config/config.go	74.0% <100.0%> (+0.6%)	⬆️
server/server.go	75.8% <100.0%> (+1.4%)	⬆️
service/pow_verifier.go	96.1% <100.0%> (-3.9%)	⬇️

... and 3 files with indirect coverage changes

[dshulyak]

i think it can start at 10ms (0.01s). otherwise there will be a lot of wasted metrics

[poszu]

I think 10ms is a little high as I observe latencies < 10ms. Changed min to 1ms.