index: add index_read_view pointer to index_read_view_iterator

We store a pointer to index read view in each implementation class,
anyway. Let's move it to the base class - this way we'll be able to
access space read view format (added later) from the code that uses
a read view iterator.

Needed for https://github.com/tarantool/tarantool-ee/issues/207

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring