In this commit, we introduce two new interfaces: SpendHintCache and
ConfirmHintCache for a height hint cache. The SpendHintCache is
responsible for maintaining the earliest height at which an outpoint
could have been spent within the chain, while the ConfirmHintCache is
responsible for maintaining the earliest height at which a transaction
confirms within the chain. We also add an implementation of these
interfaces with a single struct HeightHintCache, backed by a channeldb
instance, which will cary the duties of the interfaces above.
