Enhance pg_log_backend_memory_contexts() for auxiliary processes.

Enhance pg_log_backend_memory_contexts() for auxiliary processes.

Previously pg_log_backend_memory_contexts() could request to
log the memory contexts of backends, but not of auxiliary processes
such as checkpointer. This commit enhances the function so that
it can also send the request to auxiliary processes. It's useful to
look at the memory contexts of those processes for debugging purpose
and better understanding of the memory usage pattern of them.

Note that pg_log_backend_memory_contexts() cannot send the request
to logger or statistics collector. Because this logging request
mechanism is based on shared memory but those processes aren't
connected to that.

Author: Bharath Rupireddy
Reviewed-by: Vignesh C, Kyotaro Horiguchi, Fujii Masao
Discussion: https://postgr.es/m/CALj2ACU1nBzpacOK2q=a65S_4+Oaz_rLTsU1Ri0gf7YUmnmhfQ@mail.gmail.com