Remove unnecessary memcpy when reading WAL record fitting on page

Remove unnecessary memcpy when reading WAL record fitting on page

When reading a WAL record, its contents are copied into an intermediate
buffer.  However, doing so is not necessary if the record fits fully
into the current page, saving one memcpy for each such record.  The
allocation handling of the intermediate buffer is also now done only
when a record crosses a page boundary, shaving some extra cycles when
reading a WAL record.

Author: Andrey Lepikhov
Reviewed-by: Kyotaro Horiguchi, Heikki Linnakangas
Discussion: https://postgr.es/m/c2ea54dd-a1d3-80eb-ddbf-7e6f258e615e@postgrespro.ru