lua: add type of operation to space trigger parameters

Add the type of operation which is being executed to before_replace and
on_replace triggers.

Closes #4099

@TarantoolBot document
Title: new parameter for space before_replace and on_replace triggers
Now before_replace and on_replace triggers accept an additional
parameter: the type of operation that is being executed.
(INSERT/REPLACE/DELETE/UPDATE/UPSERT)
For example, a trigger function may now look like this:
```
function before_replace_trig(old, new, space_name, op_type)
    if op_type == 'INSERT' then
	return old
    else
	return new
    end
end
```
And will restrict all INSERTs, but allow REPLACEs, UPSERTs, DELETEs and
UPDATEs.