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.

lnrpc/wtclientrpc/wtclient.proto

@@ -87,10 +87,17 @@ message TowerSession {
     uint32 max_backups = 3;
 
     /*
+    Deprecated, use sweep_sat_per_vbyte.
     The fee rate, in satoshis per vbyte, that will be used by the watchtower for
     the justice transaction in the event of a channel breach.
     */
-    uint32 sweep_sat_per_byte = 4;
+    uint32 sweep_sat_per_byte = 4 [deprecated = true];
+
+    /*
+    The fee rate, in satoshis per vbyte, that will be used by the watchtower for
+    the justice transaction in the event of a channel breach.
+    */
+    uint32 sweep_sat_per_vbyte = 5;
 }
 
 message Tower {
@@ -172,8 +179,15 @@ message PolicyResponse {
     uint32 max_updates = 1;
 
     /*
+    Deprecated, use sweep_sat_per_vbyte.
     The fee rate, in satoshis per vbyte, that will be used by watchtowers for
     justice transactions in response to channel breaches.
     */
-    uint32 sweep_sat_per_byte = 2;
+    uint32 sweep_sat_per_byte = 2 [deprecated = true];
+
+    /*
+    The fee rate, in satoshis per vbyte, that will be used by watchtowers for
+    justice transactions in response to channel breaches.
+    */
+    uint32 sweep_sat_per_vbyte = 3;
 }