Allow chunker to traverse non-autoincrementing BIGINT columns

LHM requires the `id` column to be auto-incrementing. It never finishes when the column has large gaps between ids. This patch adds an `:autoincrementing => false` option that then executes a query to traverse successfully over an id column that has large gaps.

It executes a query like:

```sql
select max(id) from
(
select id from sites
where id > 1258013907163494815
order by id
limit 10
) as ids;
```

In order to find an appropriate range of ids for the next chunk of rows to migrate.
Update readme with :autoincrementing option