chainntnfs: move cache implementation to channeldb

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).

chainntnfs/errors.go

@@ -0,0 +1,18 @@
+package chainntnfs
+
+import "errors"
+
+var (
+	// ErrCorruptedHeightHintCache indicates that the on-disk representation
+	// has altered since the height hint cache instance was initialized.
+	ErrCorruptedHeightHintCache = errors.New("height hint cache has been " +
+		"corrupted")
+
+	// ErrSpendHintNotFound is an error returned when a spend hint for an
+	// outpoint was not found.
+	ErrSpendHintNotFound = errors.New("spend hint not found")
+
+	// ErrConfirmHintNotFound is an error returned when a confirm hint for a
+	// transaction was not found.
+	ErrConfirmHintNotFound = errors.New("confirm hint not found")
+)