client.pool: Optimise getConnForNode for most likely execution path.

Connection is more often established than not, so do a first lookup without
locking, and only acquire it if it misses. Then do a second lookup in case
another thread also established connection, and connect if it still misses.

neo/client/pool.py

@@ -143,10 +143,16 @@ class ConnectionPool(object):
         If no connection exists, create a new one"""
         if node.isRunning():
             uuid = node.getUUID()
-            self.connection_lock_acquire()
             try:
                 # Already connected to node
                 return self.connection_dict[uuid]
+            except KeyError:
+                self.connection_lock_acquire()
+                try:
+                    # Second lookup, if another thread initiated connection
+                    # while we were waiting for connection lock.
+                    try:
+                        return self.connection_dict[uuid]
                     except KeyError:
                         if len(self.connection_dict) > self.max_pool_size:
                             # must drop some unused connections