use >1 asks session connections

Gets us better throughput when polling multiple endpoints (eg. option
and stock quotes simultaneously) since slower round trip request won't
block faster ones when using multiple connections.

piker/brokers/questrade.py

@@ -212,7 +212,10 @@ class Client:
         self,
         config: dict,
     ):
-        self._sess = asks.Session()
+        # use 2 connections per streaming endpoint (stocks, opts)
+        # TODO: when we have more then one account key then this should scale
+        # linearly with that.
+        self._sess = asks.Session(connections=4)
         self.api = _API(self)
         self._conf = config
         self._is_practice = _use_practice_account or (