Use the enum value CRS_EXPORT_SNAPSHOT instead of "0"

Hi,

In the function WalReceiverMain, when the function walrcv_create_slot is called,
the fourth parameter is assigned the value "0" instead of the enum value
"CRS_EXPORT_SNAPSHOT". I think it would be better to use the corresponding enum
value.

Attach the patch to change this point.

Regards,
Wang wei