The _txn_find() must not stop at the pack boundary when it is called
by restore().  It was originally written for _txn_undo() which isn't
supposed to undo to a transaction across a pack.  But it should be
legal to restore() a transaction with a reference to a data record in
a transaction that was packed.

Fix by adding stop_at_pack flag to _txn_find() and add tests of this
behavior for FileStorage.