Configure batching of updates and deletes

[samrjenkins]

We have found that the performance of this gem can be improved through the use of batching.

I'd love to submit a PR that allows the gem to be configured with a batch size. Here are the two trivial changes we have made to our fork which batch the updates and deletions:

• team-florence@2283808
• team-florence@1063cfb

For our particular project, a batch size of 10,000 was most performant. I imagine the best implementation would not be a hardcoded value but instead to allow the value to be set via gem config. In this way, any project using the gem can specify their preferred batch size.

Keen to hear your thoughts!