vinyl: use vinyl iterators virtuality to remove 'if' in next()

The first reason of the patch is that vinyl iterators are virtual
already, and 'if's about constant index attributes (like index->id)
can be replaced by new next() implementation. Now in next() index->id
is checked to detect necessity of primary index lookup.

Lets split next() in 2 functions: primary_next() and secondary_next()
to remove 'if'.

The second reason, that in #2129 logic of secondary index lookup
complicates a lot. For example, there is raw idea to do not add
statements into a cache before looking up in a primary index, because
after #2129 any tuple, read from a secondary index, can be dirty.

Needed for #2129