This patch adds support for Backup Lock. Backup Lock allows all kind of DML
on InnoDB tables and blocks all operations that could cause an inconsistent backup,
if done during a backup operation.

To support Backup Lock the following changes were done:

  * the new flag CF_ACQUIRE_BACKUP_LOCK introduced in sql_command_flags
    to mark statements that require acquiring of BACKUP LOCK during its execution;

  * New mdl namespace BACKUP_LOCK introduced to support acquire/release of
    new kind of mdl lock - the Backup Lock. This kind of mdl is a scoped lock;

  * Added support for the new statements LOCK INSTANCE FOR BACKUP, UNLOCK INSTANCE;

  * Introduced the new system privilege BACKUP_ADMIN required to execute the statements
    LOCK INSTANCE FOR BACKUP, UNLOCK INSTANCE;

  * The new service API mysql_backup_lock was provided.