Refactor pgstat_prepare_io_time() with an input argument instead of a GUC

Refactor pgstat_prepare_io_time() with an input argument instead of a GUC

Originally, this routine relied on track_io_timing to check if a time
interval for an I/O operation stored in pg_stat_io should be initialized
or not.  However, the addition of WAL statistics to pg_stat_io requires
that the initialization happens when track_wal_io_timing is enabled,
which is dependent on the code path where the I/O operation happens.

Author: Nazir Bilal Yavuz
Discussion: https://postgr.es/m/CAN55FZ3AiQ+ZMxUuXnBpd0Rrh1YhwJ5FudkHg=JU0P+-W8T4Vg@mail.gmail.com