lnrpc+lncli: deprecate sat_per_byte and add sat_per_vbyte

This commit deprecates/replaces the old field `sat_per_byte` with
`sat_per_vbyte`. While the old field suggests sat per byte, it’s
actually using sat per virtual byte. We use the Hidden param to hide all
the deprecated flags. These flags won't show up in help menu onwards,
while stay valid that can be passed from cli. Thus bash scripts
referencing these fields won't be broken.

docs/watchtower.md

@@ -209,7 +209,7 @@ To obtain information about the watchtower's sessions, users can use the
                         "num_backups": 0,
                         "num_pending_backups": 0,
                         "max_backups": 1024,
-                        "sweep_sat_per_byte": 10
+                        "sweep_sat_per_vbyte": 10
                 }
         ]
 }