lncfg: split off protocol options into normal and legacy, normal reqs no build tag

In this commit, we split off the protocol options into a normal and
legacy sub-config. The legacy sub-config protected by a built tag, and
will only be populated if thet tag is set. Legacy options now have a
`legacy` prefix. So `--protocol.legacyonion` is now `--protocol.onion`,
and `--protocol.committweak`, is now `--protocol.legacy.committweak`.

We also create a new experimental protocol feature sub-config for newer
features that may not yet been fully complete, so they require a build
tag.

lncfg/protocol.go

@@ -0,0 +1,15 @@
+package lncfg
+
+// ProtocolOptions is a struct that we use to be able to test backwards
+// compatibility of protocol additions, while defaulting to the latest within
+// lnd, or to enable experimental protocol changes.
+type ProtocolOptions struct {
+	// LegacyProtocol is a sub-config that houses all the legacy protocol
+	// options.  These are mostly used for integration tests as most modern
+	// nodes shuld always run with them on by default.
+	LegacyProtocol `group:"legacy" namespace:"legacy"`
+
+	// ExperimentalProtocol is a sub-config that houses any experimental
+	// protocol features that also require a build-tag to activate.
+	ExperimentalProtocol
+}