GODRIVER-1695 Add timeout to IsMaster call in heartbeat

Author: sgn1

x/mongo/driver/topology/server.go

@@ -502,7 +502,11 @@ func (s *Server) heartbeat(conn *connection) (description.Server, *connection) {
 				NewIsMaster().
 				ClusterClock(s.cfg.clock).
 				Deployment(driver.SingleConnectionDeployment{initConnection{conn}})
-			err = op.Execute(ctx)
+
+			timeoutCtx, cancel := context.WithTimeout(ctx, s.cfg.heartbeatTimeout)
+			defer cancel()
+
+			err = op.Execute(timeoutCtx)
 			if err == nil {
 				tmpDesc := op.Result(s.address)
 				descPtr = &tmpDesc