go/badger: enable truncate to recover from corrupted value log file

[ptrus]

Encountered corrupted badger value log file when running txsource tests with node restarts. Apparently badger is not at all resilient to crashes unless the truncate option is set on (which could result in data loss, but i think this should be safe as this means the crash happened while it was being written to the value log, so not yet committed).

{"caller":"node.go:741","err":"tendermint: failed to create node: tendermint/db/badger: failed to open database: During db.vlog.open: Value log truncate required to run DB. This might result in data loss","level":"error","module":"oasis-node","msg":"failed to start tendermint service","ts":"2020-02-27T09:42:18.599026044Z"}

This was also default behavior before an configuration option was added: dgraph-io/badger#452 dgraph-io/badger#740

[codecov]

Codecov Report

Merging #2732 into master will increase coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2732      +/-   ##
=========================================
+ Coverage   62.72%   62.9%   +0.18%     
=========================================
  Files         381     381              
  Lines       35753   35757       +4     
=========================================
+ Hits        22426   22494      +68     
+ Misses      10512   10434      -78     
- Partials     2815    2829      +14

Impacted Files	Coverage Δ
go/storage/mkvs/urkel/db/badger/badger.go	69.3% <100%> (+0.06%)	⬆️
go/runtime/localstorage/localstorage.go	56.14% <100%> (+0.78%)	⬆️
go/common/persistent/persistent.go	84.31% <100%> (+0.31%)	⬆️
go/runtime/history/db.go	82.64% <100%> (+0.14%)	⬆️
go/consensus/tendermint/db/badger/badger.go	68.21% <100%> (+0.12%)	⬆️
go/worker/storage/service_external.go	47.31% <0%> (-6.46%)	⬇️
go/runtime/committee/nodes.go	80.64% <0%> (-2.16%)	⬇️
go/worker/compute/txnscheduler/committee/node.go	62.34% <0%> (-1.86%)	⬇️
go/worker/registration/worker.go	66.34% <0%> (-0.49%)	⬇️
go/oasis-node/cmd/root.go	86.48% <0%> (-0.36%)	⬇️
... and 20 more

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