In this commit, we convert the `JusticeKit` struct to an interface.
Then, we add two implementations of that interface:
1) The `legacyJusticeKit` which implements all the methods of
`JusticeKit`
2) The `anchorJusticKit` which wraps the `legacyJusticeKit` and just
re-implements the `ToRemoteOutputSpendInfo` method since.
Modifies the bbolt and mock tower databases to only accept blobs that
are the expected size of the session's blob type. This prevents resource
exhaustion attacks where a client may provide disproportionately large
encrypted blob, even though all supported blob types are of fixed-size.
