Add the thd_kill_statement service that provides read access to the thd->killed flag.

This service will obsolete thd_killed() from the plugin.h, we'll remove this declaration, but keep the function in the sql_class.cc until the text major plugin API version change.

The service can look like this:

enum thd_kill_levels {

  THD_IS_NOT_KILLED=0,

  THD_ABORT_SOFTLY=50, /**< abort when possible, don't leave tables corrupted */

  THD_ABORT_ASAP=100, /**< abort asap */

};

#define thd_killed(THD)   (thd_kill_level(THD) == THD_ABORT_ASAP)

enum thd_kill_levels thd_kill_level(const MYSQL_THD);

it maps the internal bitmap of various kill states into few levels that a plugin needs to care about.