key-manager: new unsecret state key

[pro-wh]

blocks https://github.com/oasislabs/runtime-ethereum/issues/606

The plan here:

1. Make it possible to configure the set of keys to use for the key manager.
2. Reprovision a new set of explicitly unsecret keys to use by default.
3. Configure production to use the old keys.

[Yawning]

The approach looks reasonable enough to me. Especially for placeholders, the MRAE symmetric key can be anything as long as it's the right size.

[codecov]

Codecov Report

Merging #1504 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1504      +/-   ##
==========================================
- Coverage    54.4%   54.37%   -0.03%     
==========================================
  Files         160      160              
  Lines       14371    14371              
==========================================
- Hits         7818     7814       -4     
- Misses       5801     5804       +3     
- Partials      752      753       +1

Impacted Files	Coverage Δ
go/scheduler/alg/greedy_subgraphs_scheduler.go	84.21% <0%> (-5.27%)	⬇️

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[codecov]

Codecov Report

Merging #1504 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1504      +/-   ##
==========================================
- Coverage   55.62%   55.49%   -0.13%     
==========================================
  Files         161      161              
  Lines       14376    14376              
==========================================
- Hits         7996     7978      -18     
- Misses       5605     5624      +19     
+ Partials      775      774       -1

Impacted Files	Coverage Δ
go/client/client.go	59.42% <0%> (-8.7%)	⬇️
go/common/node/node.go	20.28% <0%> (-2.84%)	⬇️

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[pro-wh]

Thanks for the review. Gonna rebase for the gx fixes. Let's see if CI detects any regressions, with the default configuration.