PE-1656 - Made 'wait_timeout' emulated only so that it does not get
pushed down to the backend sites. Its value is used in SSConnectio
timeout check.
Added a new DVE read-only variable 'backend_wait_timeout'. Its value
gets pushed down to all p/d-sites (MysqlConnection).

Made MysqlConnection pick up its default variables from global values of
the existing KnownVariables rather than using hardcoded constants.