box: rework iterator_position entity

There is a big caveat in current implementation of position - it is
represented as a string with unprintable chars in Lua - if user will
need to save it to another database or pass to another client,
unprintable chars can be affected. That is why we decided to redesign
position entity - now it is just a key with MP_ARRAY header, encoded to
base64 format.

NO_CHANGELOG=redesign of unreleased feature
NO_DOC=later

Part of #7639