get_imported_privkey_branch: use O(m+n) algorithm instead of O(m*n)

The algorithm in get_imported_privkey_branch was O(m*n): for each
imported path, it was iterating over the entire set of UTXOs. Rewrite
the algorithm to make one pass over the set of UTXOs up front to compute
the balance of each script (O(m)) and then, separately, one pass over
the set of imported paths to pluck out the balance for each path (O(n)).

src/jmclient/wallet_utils.py

@@ -7,7 +7,7 @@ import sys
 from datetime import datetime, timedelta
 from optparse import OptionParser
 from numbers import Integral
-from collections import Counter
+from collections import Counter, defaultdict
 from itertools import islice, chain
 from jmclient import (get_network, WALLET_IMPLEMENTATIONS, Storage, podle,
     jm_single, WalletError, BaseWallet, VolatileStorage,
@@ -403,15 +403,15 @@ def get_tx_info(txid, tx_cache=None):
 
 def get_imported_privkey_branch(wallet_service, m, showprivkey):
     entries = []
+    balance_by_script = defaultdict(int)
+    for data in wallet_service.get_utxos_at_mixdepth(m,
+        include_disabled=True).values():
+        balance_by_script[data['script']] += data['value']
     for path in wallet_service.yield_imported_paths(m):
         addr = wallet_service.get_address_from_path(path)
         script = wallet_service.get_script_from_path(path)
-        balance = 0.0
-        for data in wallet_service.get_utxos_at_mixdepth(m,
-            include_disabled=True).values():
-            if script == data['script']:
-                balance += data['value']
-        status = ('used' if balance > 0.0 else 'empty')
+        balance = balance_by_script.get(script, 0)
+        status = ('used' if balance else 'empty')
         if showprivkey:
             wip_privkey = wallet_service.get_wif_path(path)
         else: