vinyl: sanitize full/empty key stmt detection

Historically, we use tuple_field_count to check whether a statement
represents an empty key (match all) or a full key (point lookup): if
the number of fields in a tuple is greater than or equal to the number
of parts in a key definition, it can be used as a full key; if the
number of fields is zero, then the statement represents an empty key.

While this used to be correct not so long ago, appearance of JSON
indexes changed the rules of the game: now a tuple can have nested
indexed fields so that the same field number appears in the key
definition multiple times. This means tuple_field_count can be less
than the number of key parts and hence the full key check won't work
for a statement representing a tuple.

Actually, any tuple in vinyl can be used as a full key as it has all
key parts by definition, there's no need to use tuple_field_count for
such statements - we only need to do that for statements representing
keys. Keeping that in mind, let's introduce helpers for checking
whether a statement can be used as a full/empty key and use them
throughout the code.