go/consensus/tendermint: Don't use UnsafeSigner

[Yawning]

No, that's German for "The UnsafeSigner, The"

[codecov]

Codecov Report

Merging #2670 into master will increase coverage by 0.01%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #2670      +/-   ##
==========================================
+ Coverage   63.06%   63.07%   +0.01%     
==========================================
  Files         365      365              
  Lines       34587    34586       -1     
==========================================
+ Hits        21812    21815       +3     
- Misses      10056    10060       +4     
+ Partials     2719     2711       -8

Impacted Files	Coverage Δ
go/common/crypto/signature/signer.go	92.59% <ø> (ø)	⬆️
...ommon/crypto/signature/signers/file/file_signer.go	56.97% <ø> (-0.98%)	⬇️
go/consensus/tendermint/tendermint.go	65.46% <100%> (-2.13%)	⬇️
go/consensus/tendermint/seed.go	74.11% <100%> (+1.39%)	⬆️
go/consensus/tendermint/crypto/signature.go	81.81% <66.66%> (-18.19%)	⬇️
go/consensus/api/grpc.go	57.02% <0%> (-6.62%)	⬇️
go/common/grpc/proxy/proxy.go	58.87% <0%> (-5.61%)	⬇️
go/storage/metrics.go	73.21% <0%> (-2.68%)	⬇️
go/storage/mkvs/urkel/lookup.go	72.72% <0%> (-2.28%)	⬇️
... and 11 more

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