There are separate flags DBUG_OFF for disabling the DBUG facility
and ENABLED_DEBUG_SYNC for enabling the DEBUG_SYNC facility.
Let us allow debug builds without DEBUG_SYNC.
Note: For CMAKE_BUILD_TYPE=Debug, CMakeLists.txt will continue to
define ENABLED_DEBUG_SYNC.
* rename in_subquery_conversion_threshold to in_predicate_conversion_threshold
* make it debug-only, hide from users
* change from ulong to uint - same type and range on all architectures