API CHANGE: Moved the meat of SQLQuery's SQL generation into Database::sqlQueryToString(), to be overloaded by different database implementations.
API CHANGE: Moved the implementation of ClassInfo::hasTable() and ClassInfo::getValidSubClasses() into Database::hasTable() and Database::enumValuesForField(), to be overloaded by different database implementations.
git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/sapphire/trunk@66422 467b73ca-7a2a-4603-9d3b-597d59a354a9