memtx: factor out memtx_prepare_read_view_tuple

We can't return tuples retrieved from a consistent index read view as
is. We need to filter out dirty tuples and decompress compressed tuples.
Let's introduce a helper function for this. Currently, this helper
function is used only in the ALL iterator implementation of the tree and
hash indexes. Later, we will use it in all other iterator types, which
will be implemented in the EE repository.

Needed for https://github.com/tarantool/tarantool-ee/issues/197

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring