Rename some support functions for pgstat* views.

Rename some support functions for pgstat* views.

Presently, pgstat_fetch_stat_beentry() accepts a session's backend
ID as its argument, and pgstat_fetch_stat_local_beentry() accepts a
1-based index in an internal array as its argument.  The former is
typically used wherever a user must provide a backend ID, and the
latter is usually used internally when looping over all entries in
the array.  This difference was first introduced by d7e39d72ca.
Before that commit, both functions accepted a 1-based index to the
internal array.

This commit renames these two functions to make it clear whether
they use the backend ID or the 1-based index to look up the entry.
This is preparatory work for a follow-up change that will introduce
a function for looking up a LocalPgBackendStatus using a backend
ID.

Reviewed-by: Ian Barwick, Sami Imseih, Michael Paquier, Robert Haas
Discussion: https://postgr.es/m/CAB8KJ%3Dj-ACb3H4L9a_b3ZG3iCYDW5aEu3WsPAzkm2S7JzS1Few%40mail.gmail.com
Backpatch-through: 16