graph/db: validate edge policy TLV data before persisting

In this commit, we start validating the extra opaque data of a channel
edge policy before persisting it. We just check that the data is valid
TLV.

NOTE: we recently [started
validating](https://github.com/lightningnetwork/lnd/commit/1410a0949d555e14de11faf81a17fe64917505e3)
this at the lnwire level. So really, no new update will reach the DB
layer without this already being checked. But we check it again here so
that the DB API behaves correctly as its own unit.

graph/db/errors.go

@@ -12,6 +12,10 @@ var (
 	ErrEdgePolicyOptionalFieldNotFound = fmt.Errorf("optional field not " +
 		"present")
 
+	// ErrParsingExtraTLVBytes is returned when we attempt to parse
+	// extra opaque bytes as a TLV stream, but the parsing fails.
+	ErrParsingExtraTLVBytes = fmt.Errorf("error parsing extra TLV bytes")
+
 	// ErrGraphNotFound is returned when at least one of the components of
 	// graph doesn't exist.
 	ErrGraphNotFound = fmt.Errorf("graph bucket not initialized")