box/lua: switch box.iproto.override() to event triggers

Switch the legacy `box.iproto.override()' interface to the newly introduced
event triggers. This change is mostly not user-visible, except:
 - Now it can be called before `box.cfg{}';
 - Now request type can be set as a string;
 - Some changes in error messages;
 - The "overriding does not support ... request type" error is logged,
   rather than raised;
 - The internal trigger is visible via the `trigger' module.

If some request type is overridden by both interfaces (legacy
`box.iproto.override()' and new `trigger.set()'), the order of invocation
of the handlers is unspecified.

Closes #8138

NO_DOC=internal