Use constrained_check<> classes on kernel sysvars for Drizzle

The global sys_var variables in drizzled/sys_var.cc need to be migrated to use constrained_check<> instead of max/min/default values from the option struct

Refactor system variables system for Drizzle

Refactor the sys_var and plugin_sys_var system to be a single, C++, typesafe system with a pluggable implementation.