lightningd: make minimum feerate significantly lower.

estimatesmartfee 4 ECONOMICAL was too high for lnd, so drop it, with some
increased security risk.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/peer_htlcs.c

@@ -1568,11 +1568,12 @@ void notify_feerate_change(struct lightningd *ld)
 		if (!peer->owner)
 			continue;
 
+		/* FIXME: low bound is probably too low. */
 		msg = towire_channel_feerates(peer,
 					      get_feerate(ld->topology,
 							  FEERATE_IMMEDIATE),
 					      get_feerate(ld->topology,
-							  FEERATE_NORMAL),
+							  FEERATE_NORMAL) / 2,
 					      get_feerate(ld->topology,
 							  FEERATE_IMMEDIATE)
 					      * 5);