This commit was previously split into the following parts to ease
review:
- 2d746f68: replace imports
- 4008f0fd: use ecdsa.Signature
- 849e33d1: remove btcec.S256()
- b8f6ebbd: use v2 library correctly
- fa80bca9: bump go modules
In this commit, we migrate all wire messages in the database from the
`legacy` to the `current` encoding.
This affects the way we write the `CommitDiff` and `LogUpdates` struct
to disk. We also need to migrate the network results bucket in the
switch as it includes a wire message without a length prefix.
