Fix BOX_RETURN_TUPLE logic for on_request trigger
Fix tests
Rename on_request/remove_on_request to set_on_request/clear_on_request
Move on_request code to box.cc
Remove unused return value from on_requets function
Rework Lua bindings for on_request

 * Remove RT_FIRST/RT_USER/RT_LAST logic
 * Use trigger object (userdata) instead of trigger id
 * Add support for trigger context
Remove request_trigger->data member
Pass context as the first argument to on_request trigger
Replace process_replica with an on_request trigger
Move Lua bindings for struct request to box_lua_request.h(.cc)
Minor fixes in on_request_trigger code
Add more tests for on_request trigger
Add CALL arguments to Lua bindings for struct request