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).

lnwallet/test/test_interface.go

@@ -3055,10 +3055,10 @@ func TestLightningWallet(t *testing.T, targetBackEnd string) {
 	tempDir := t.TempDir()
 	db, err := channeldb.Open(tempDir)
 	require.NoError(t, err, "unable to create db")
-	testCfg := chainntnfs.CacheConfig{
+	testCfg := channeldb.CacheConfig{
 		QueryDisable: false,
 	}
-	hintCache, err := chainntnfs.NewHeightHintCache(testCfg, db.Backend)
+	hintCache, err := channeldb.NewHeightHintCache(testCfg, db.Backend)
 	require.NoError(t, err, "unable to create height hint cache")
 	blockCache := blockcache.NewBlockCache(10000)
 	chainNotifier, err := btcdnotify.New(