Identify walsenders in pg_stat_activity

Following 8299471c walsender procs are now visible in pg_stat_activity.
Set query to ‘walsender’ for walsender procs to allow them to be identified.

Discussion:CAB7nPqS8c76KPSufK_HSDeYrbtg+zZ7D0EEkjeM6txSEuCB_jA@mail.gmail.com

Michael Paquier, issue raised by Fujii Masao, reviewed by Tom Lane