lightningd: clarify uses of dynamic (mempool) feerate floor, and static.

We have the FEERATE_FLOOR constant if you don't care, but usually you want
to use the current bitcoind lower limit, so call get_feerate_floor()
(which is currently the same, but coming!).

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

bitcoin/feerate.c

@@ -1,8 +1,12 @@
 #include "config.h"
+#include <assert.h>
 #include <bitcoin/feerate.h>
 
 u32 feerate_from_style(u32 feerate, enum feerate_style style)
 {
+	/* Make sure it's called somewhere! */
+	assert(feerate_floor_check() == FEERATE_FLOOR);
+
 	switch (style) {
 	case FEERATE_PER_KSIPA:
 		return feerate;