Key-aware batching function

I have a CombineFn for which add_input has very large overhead. I would like to batch the incoming elements into a large batch before each call to add_input to reduce this overhead. In other words, I would like to do something like: 

`elements | GroupByKey() | BatchElements() | CombineValues(MyCombineFn())`

Unfortunately, BatchElements is not key-aware, and can't be used after a GroupByKey to batch elements per key. I'm working around this by doing the batching within CombineValues, which makes the CombineFn rather messy. It would be nice if there were a key-aware BatchElements transform which could be used in this context.

Imported from Jira [BEAM-3737](https://issues.apache.org/jira/browse/BEAM-3737). Original Jira may contain additional context.
Reported by: chuanyu.

Start by reading the existing BatchElements, GroupByKey, and CombineValues transforms and the CombineFn add_input contract. Determine how batching can preserve per-key grouping and define the expected behavior for the key-aware transform, including how it should be tested. Done means a usable key-aware batching transform that fits the shown pipeline and has coverage for batching elements per key.

Written by the indexing model from the issue text.