lnwallet: increase legacy fee limit threshold to 1k sats

In this commit, we increase the legacy fee limit threshold (the amount
below which we'll allow 100% of funds to go to fees for the non-v2 RPC
calls) from 50 sats to 1k sats.

lnrpc/lightning.proto

@@ -754,7 +754,7 @@ message SendRequest {
     This value can be represented either as a percentage of the amount being
     sent, or as a fixed amount of the maximum fee the user is willing the pay to
     send the payment. If not specified, lnd will use a default value of 100%
-    fees for small amounts (<=50 sat) or 5% fees for larger amounts.
+    fees for small amounts (<=1k sat) or 5% fees for larger amounts.
     */
     FeeLimit fee_limit = 8;
 
@@ -2576,7 +2576,7 @@ message QueryRoutesRequest {
     This value can be represented either as a percentage of the amount being
     sent, or as a fixed amount of the maximum fee the user is willing the pay to
     send the payment. If not specified, lnd will use a default value of 100%
-    fees for small amounts (<=50 sat) or 5% fees for larger amounts.
+    fees for small amounts (<=1k sat) or 5% fees for larger amounts.
     */
     FeeLimit fee_limit = 5;