Allow walreceiver configuration to change on reload

Allow walreceiver configuration to change on reload

The parameters primary_conninfo, primary_slot_name and
wal_receiver_create_temp_slot can now be changed with a simple "reload"
signal, no longer requiring a server restart.  This is achieved by
signalling the walreceiver process to terminate and having it start
again with the new values.

Thanks to Andres Freund, Kyotaro Horiguchi, Fujii Masao for discussion.

Author: Sergei Kornilov <sk@zsrv.org>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/19513901543181143@sas1-19a94364928d.qloud-c.yandex.net