+ /**
+ * Gets estimated satoshis of fee required per 1000 Weight-Units.
+ *
+ * Must be no smaller than 253 (ie 1 satoshi-per-byte rounded up to ensure later round-downs
+ * don't put us below 1 satoshi-per-byte).
+ *
+ * This translates to:
+ * satoshis-per-byte * 250
+ * ceil(satoshis-per-kbyte / 4)
+ */