Rename routines for write/read of pgstats file

Rename routines for write/read of pgstats file

This commit renames write_chunk and read_chunk to respectively
pgstat_write_chunk() and pgstat_read_chunk(), along with the *_s
convenience macros.

These are made available for plug-ins, so as any code that decides to
write and/or read stats data can rely on a single code path for this
work.

Extracted from a larger patch by the same author.

Author: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0s9SDOu+Z6veoJCHWk+kDeTktAtC-KY9fQ9Z6BJdDUirQ@mail.gmail.com