vinyl: don't use IPROTO_SELECT type for key statements

To differentiate between key and tuple statements in comparators, we set
IPROTO_SELECT type for key statements. As a result, we can't use key
statements in the run iterator directly although secondary index runs do
store statements in key format. Instead we create surrogate tuples
filling missing fields with NULLs. This won't play nicely with multikey
indexes so we need to teach iterators to deal with statements in key
format. The first step in this direction is dropping IPROTO_SELECT in
favor of identifying key statements by format.