Make code more robust

[kcalvinalvin]

Plan

utcd code is was more focused on just getting a proof-of-concept utreexo node out there. Now that's happened, start working on making the code more robust and prep stuff so that the end user would be able to use it.

Goal

• Make all tests pass with cgo.
• Merge overdue PRs in btcd.
• Have every Utreexo related commit in a package as just one commit, based on the branch with btcd with overdue PRs merged.

Extras/wish list:

• Make utcd storage compatible with Bitcoin Core (a user should be able to run core then run utcd).