keymanager: Add support for master secret generations

[peternose]

Master secret rotation, 1st part:

• Refactor key manager's init method to be able to support multiple generations of the master secret.

[codecov]

Codecov Report

Merging #5198 (cf872c2) into master (cf5e508) will increase coverage by 0.13%.
The diff coverage is n/a.

❗ Current head cf872c2 differs from pull request most recent head c5a594a. Consider uploading reports for the commit c5a594a to get more accurate results

@@            Coverage Diff             @@
##           master    #5198      +/-   ##
==========================================
+ Coverage   61.47%   61.60%   +0.13%     
==========================================
  Files         512      512              
  Lines       54051    54055       +4     
==========================================
+ Hits        33227    33303      +76     
+ Misses      16611    16525      -86     
- Partials     4213     4227      +14     

Impacted Files	Coverage Δ
go/roothash/api/sanity_check.go	60.00% <0.00%> (-8.00%)	⬇️
go/common/cbor/codec.go	78.37% <0.00%> (-5.41%)	⬇️
go/runtime/host/helpers.go	66.03% <0.00%> (-3.78%)	⬇️
go/runtime/host/protocol/connection.go	62.03% <0.00%> (-3.01%)	⬇️
go/consensus/tendermint/abci/mux.go	44.44% <0.00%> (-2.63%)	⬇️
go/consensus/tendermint/apps/roothash/messages.go	71.42% <0.00%> (-2.39%)	⬇️
go/oasis-node/cmd/node/node.go	51.40% <0.00%> (-2.19%)	⬇️
go/p2p/peermgmt/peermgr.go	84.87% <0.00%> (-1.96%)	⬇️
go/p2p/peermgmt/connector.go	98.16% <0.00%> (-1.84%)	⬇️
go/oasis-node/cmd/stake/delegations.go	78.99% <0.00%> (-1.69%)	⬇️
... and 30 more

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