This commit moves the `HeightHintCache` implementation to the
`channeldb` package and inverts the dependency relation between
`chainntnfs` and `channeldb`.
Many packages depend on channeldb for type definitions,
interfaces, etc. `chainntnfs` is an example of that. `chainntnfs`
defines the `SpendHintCache` and `ConfirmHintCache` interfaces but
it also implments them (`HeightHintCache` struct). The implementation
uses logic that should not leak from channeldb (ex: bucket paths).
This makes our code highly coupled + it would not allow us to use any
of these interfaces in a package that is imported by `channeldb`
(circular dependency).
