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/test/test_interface.go

@@ -1836,10 +1836,10 @@ func TestInterfaces(t *testing.T, targetBackEnd string) {
 		if err != nil {
 			t.Fatalf("unable to create db: %v", err)
 		}
-		testCfg := chainntnfs.CacheConfig{
+		testCfg := channeldb.CacheConfig{
 			QueryDisable: false,
 		}
-		hintCache, err := chainntnfs.NewHeightHintCache(
+		hintCache, err := channeldb.NewHeightHintCache(
 			testCfg, db.Backend,
 		)
 		if err != nil {