synchronizer: fix race in _on_address_status

Triggering needs two consecutive scripthash status changes
in very quick succession. Client gets notification from server,
but then response to "blockchain.scripthash.get_history" will already contain
the changed-again history that has a different status.

20190627T101547.902638Z |     INFO | synchronizer.[default_wallet] | receiving history mwXtx49BCGAiy4tU1r7MBX5VVLWSdtasCL 1
20190627T101547.903262Z |     INFO | synchronizer.[default_wallet] | error: status mismatch: mwXtx49BCGAiy4tU1r7MBX5VVLWSdtasCL

electrum/synchronizer.py

@@ -147,7 +147,7 @@ class Synchronizer(SynchronizerBase):
     def _reset(self):
         super()._reset()
         self.requested_tx = {}
-        self.requested_histories = {}
+        self.requested_histories = set()
 
     def diagnostic_name(self):
         return self.wallet.diagnostic_name()
@@ -161,10 +161,10 @@ class Synchronizer(SynchronizerBase):
         history = self.wallet.db.get_addr_history(addr)
         if history_status(history) == status:
             return
-        if addr in self.requested_histories:
+        if (addr, status) in self.requested_histories:
             return
         # request address history
-        self.requested_histories[addr] = status
+        self.requested_histories.add((addr, status))
         h = address_to_scripthash(addr)
         self._requests_sent += 1
         result = await self.network.get_history_for_scripthash(h)
@@ -188,7 +188,7 @@ class Synchronizer(SynchronizerBase):
             await self._request_missing_txs(hist)
 
         # Remove request; this allows up_to_date to be True
-        self.requested_histories.pop(addr)
+        self.requested_histories.discard((addr, status))
 
     async def _request_missing_txs(self, hist, *, allow_server_not_finding_tx=False):
         # "hist" is a list of [tx_hash, tx_height] lists