Add support to resume process in case of crashes

[marcosnils]

I need to scan a huge collection and sometimes my process crashes due to OOM. It'd be nice to save the LastEvaluatedKey of each process (in a file maybe?) so if I start the process again I can indicate to resume the scan instead of starting all over

thoughts?

[amcp]

@marcosnils This could be implemented with a checkpoint table for a completely automated copy recovery solution. However, I do recommend you tune your execution parameters so that you avoid OOM as well (see my reply in #2). Pull requests are welcome.