Cleanup: Replace sscanf with strtol/strtoul in snapmgr

Hi,

The attached patch replaces sscanf with strtol and strtoul in the
ImportSnapshot helpers (parseIntFromText, parseXidFromText, and
parseVxidFromText) to improve reliability and efficiency. By utilizing
the end pointer, we can locate the next line without re-scanning the
entire string.

Additionally, this change aligns the snapshot code with the rest of
the Postgres backend, which already favors these functions for safer
parsing.

--
Regards,
Amul Sul
EDB: http://www.enterprisedb.com