+ /**Gets estimated satoshis of fee required per 1000 Weight-Units.
+ *
+ * Must return a value 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 method can be implemented with the following unit conversions:
+ * max(satoshis-per-byte * 250, 253)
+ * max(satoshis-per-kbyte / 4, 253)
+ */