Use LogRootV1 with SignLogRoot / VerifyLogRoot

[gdbelvin]

This PR continues in the theme of #958 by using the LogRootV1 defined in #1037 to create *trillian.SignedLogRoot objects via SignLogRoot. Verifying a *trillian.SignedLogRoot will also extract and return the LogRootV1 object.

Local references to trillian.SignedLogRoot that access it's fields will slowly be replaced with LogRootV1 objects.

Note that LogID is no longer a part of LogRootV1. In future PRs, LogID will be more related to the signer than the LogRootV1 object.

[codecov-io]

Codecov Report

Merging #1048 into master will decrease coverage by 0.02%.
The diff coverage is 91.37%.

@@            Coverage Diff             @@
##           master    #1048      +/-   ##
==========================================
- Coverage   62.69%   62.66%   -0.03%     
==========================================
  Files         103      103              
  Lines        8408     8413       +5     
==========================================
+ Hits         5271     5272       +1     
- Misses       2610     2613       +3     
- Partials      527      528       +1

Impacted Files	Coverage Δ
server/log_rpc_server.go	77.46% <100%> (-0.21%)	⬇️
crypto/verifier.go	50.94% <100%> (+5.11%)	⬆️
storage/tools/dump_tree/dumplib/dumplib.go	41.94% <100%> (-0.22%)	⬇️
crypto/signer.go	74.46% <100%> (+2.37%)	⬆️
log/sequencer.go	79.27% <100%> (-0.19%)	⬇️
client/log_verifier.go	73.58% <78.57%> (+0.5%)	⬆️
client/log_client.go	70% <88.88%> (-1.07%)	⬇️
util/tracker.go	96.61% <0%> (-3.39%)	⬇️

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[Martin2112]

This doesn't seem to have affected many tests. Is that OK? They are mostly internal changes I suppose.

[gdbelvin]

You might know better than me, but it seems to me like it's hitting the right number of tests.
Many of our tests mock out the signing bits don't they?

[Martin2112]

Yes there's a lot of mock signer usage and nothing much is using the signatures at the moment. It's probably OK.

[gdbelvin]

LGTM?