Move SQL-callable code related to multixacts into its own file

Move SQL-callable code related to multixacts into its own file

A patch is under discussion to add more SQL capabilities related to
multixacts, and this move avoids bloating the file more than necessary.
This affects pg_get_multixact_members().  A side effect of this move is
the requirement to add mxstatus_to_string() to multixact.h.

Extracted from a larger patch by the same author, tweaked by me.

Author: Naga Appani <nagnrik@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://postgr.es/m/CA+QeY+AAsYK6WvBW4qYzHz4bahHycDAY_q5ECmHkEV_eB9ckzg@mail.gmail.com