Find all non-snapshotted versions of a symbol that are older than a version that's at least keep_mins
        minutes old.

        Based on documents available on the secondary.

def _find_prunable_version_ids(self, symbol, keep_mins):
        """
        Find all non-snapshotted versions of a symbol that are older than a version that's at least keep_mins
        minutes old.

        Based on documents available on the secondary.
        """
        read_preference = ReadPreference.SECONDARY_PREFERRED if keep_mins > 0 else ReadPreference.PRIMARY
        versions = self._versions.with_options(read_preference=read_preference)
        query = {'symbol': symbol,
                 # Not snapshotted
                 '$or': [{'parent': {'$exists': False}}, {'parent': []}],
                 # At least 'keep_mins' old
                 '_id': {'$lt': bson.ObjectId.from_datetime(dt.utcnow()
                                                            # Add one second as the ObjectId
                                                            # str has random fuzz
                                                            + timedelta(seconds=1)
                                                            - timedelta(minutes=keep_mins)
                                                            )
                         }
                 }
        cursor = versions.find(query,
                               # Using version number here instead of _id as there's a very unlikely case
                               # where the versions are created on different hosts or processes at exactly
                               # the same time.
                               sort=[('version', pymongo.DESCENDING)],
                               # Guarantees at least one version is kept
                               skip=1,
                               projection={'_id': 1, FW_POINTERS_REFS_KEY: 1, FW_POINTERS_CONFIG_KEY: 1},
                               )
        return {v['_id']: ([bson.binary.Binary(x) for x in v.get(FW_POINTERS_REFS_KEY, [])], get_fwptr_config(v))
                for v in cursor}