WL#5197: Move @@engine_condition_pushdown to @@optimizer_switch

This represents the piece of WL#5116 which is specifically to be done in next-mr.

"set engine_condition_pushdown" will be deprecated (it exists in 5.0/1);
engine condition pushdown will be controlled by a new "set
optimizer_switch=engine_condition_pushdown=on|off".
This is to be done in next-mr, so that deprecation happens ASAP in the next GA.

- add an engine_condition_pushdown=on|off switch to @@optimizer_switch
- make sure that setting @@engine_condition_pushdown issues a warning
(WARN_DEPRECATED)
- make sure that @@optimizer_switch=engine_condition_pushdown=on|off and
@@engine_condition_pushdown=on|off influence each other.

Patches:
http://lists.mysql.com/commits/94451
http://lists.mysql.com/commits/94606