lnrpc: rename MaxHtlc to MaxHtlcMsat in lnrpc.RoutingPolicy

In this commit, we rename the new `MaxHtlc` field to `MaxHtlcMsat`. We
do this in order to adhere to the new practice to suffix each amount
related field with their proper unit.

lnrpc/rpc.proto

@@ -1597,7 +1597,7 @@ message RoutingPolicy {
     int64 fee_base_msat = 3 [json_name = "fee_base_msat"];
     int64 fee_rate_milli_msat = 4 [json_name = "fee_rate_milli_msat"];
     bool disabled = 5 [json_name = "disabled"];
-    uint64 max_htlc = 6 [json_name = "max_htlc"];
+    uint64 max_htlc_msat = 6 [json_name = "max_htlc_msat"];
 }
 
 /**

lnrpc/rpc.swagger.json

@@ -2816,7 +2816,7 @@
           "type": "boolean",
           "format": "boolean"
         },
-        "max_htlc": {
+        "max_htlc_msat": {
           "type": "string",
           "format": "uint64"
         }