Batching transformer

[johnynek]

Suppose your client is going to make a bunch of gets, but you want to make sure those are batched into blocks of at least N or after a period of at most T, and send it to multiGet.

This makes it easy for callers to continue to use the get API, but to be more efficient and block them up internally with a multiGet.

We can do the same thing on write/merge (in fact, we have some of this for merging now).