sonic_data_client: make PollStats stoppable and wire into TestMain#606

sigabrtv1-ui wants to merge 2 commits intosonic-net:mastersonic-net/sonic-gnmi:masterfrom

sigabrtv1-ui:fix/stop-poll-statssigabrtv1-ui/sonic-gnmi:fix/stop-poll-statsCopy head branch name to clipboard