tarantool

The create_snapshot_iterator index callback is used by the memtx engine
to create a consistent read view of data stored in memtx so that it can
be written to a snapshot or sent to a remote replica. We also define and
use this callback internally in vinyl to implement initial join.

Actually, there's no need to have this code wrapped in a callback in
vinyl, because it's never called from outside the vinyl internals. Let's
inline it and drop the callback for vinyl. This will simplify further
refactoring of the internal index read view API.

Needed for #7194

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring