Track tx size directly; calculate fees from that

This has several advantages.  Fee calculation is now very fast,
as we don't need to keep reserializing the tx.  Another is that
we can reason about the fees after adding a change output without
having to add it, recalculate the tx fee, and remove it again.

lib/wallet.py

@@ -24,6 +24,7 @@ import random
 import time
 import json
 import copy
+from functools import partial
 
 from util import PrintError, profiler
 
@@ -929,12 +930,10 @@ class Abstract_Wallet(PrintError):
 
         # Fee estimator
         if fixed_fee is None:
-            fee_per_kb = self.fee_per_kb(config)
-            def fee_estimator(tx):
-                return tx.estimated_fee(fee_per_kb)
+            fee_estimator = partial(Transaction.fee_for_size,
+                                    self.fee_per_kb(config))
         else:
-            def fee_estimator(tx):
-                return fixed_fee
+            fee_estimator = lambda size: fixed_fee
 
         # Change <= dust threshold is added to the tx fee
         dust_threshold = 182 * 3 * MIN_RELAY_TX_FEE / 1000