fix type clash

all important types are now in an extra header
fix mysql_option not completely renamed
correctly unload all systems at plugin shutdown
update handle tests
removed native 'mysql_current_handle'

since the cache system is now working globally (and not per-handle), there is no 'active handle' anymore
re-implement mysql_errno
small code improvement
re-implement mysql_escape_string
re-implement mysql_set_charset
re-implement mysql_get_charset
re-implement mysql_stat
protect connection pool access with mutex