fix dataiterator returning random data in some cases

Fixed two issues:
1) iterating over attributes in with vertical storage could return
junk when some other operation paged in some other data.
2) the returned string could also be in the paged area, or also
in tmpspace if stringification was done. We can't simply alloc
memory as this makes things slower, but in most cases the storage
does not matter. So we offer a new function, dataiterator_strdup(),
that makes the kv.str pointer persistent.

WARNING: this commit is an ABI change as it changes the dataiterator
structure. Sorry.