use couch_util:to_hex for rev_id generation

#hexiolist was seeing errors on a snapshot of the
production database:

```
exception exit: function_clause
   in function  decouch_reader:hexiolist/1
      called as decouch_reader:hexiolist(<<"379909253">>)
```

The issue in particular was that the rev_id that
came back was smaller than expected (not 16 bytes
md5-ish).