iproto: add IPROTO_NOP request type

To implement space:before_replace trigger, we need to introduce a new
request type for bumping LSN, because the new trigger may turn any DML
operation into a no-op. Let's call it IPROTO_NOP. It is treated as DML
(passed to apply_row, etc), but it is ignored by space_execute_dml() and
so doesn't actually modify anyting, only bumps LSN on the server. The
new request type has name "NOP" (for xlog reader), however it isn't
reported via box.stat().

Needed for #2993