Expose a stream api and implement grouper in terms of streams.

[spion]

Instead of a row-emitting eventemitter, expose a stream. The benefits of streams are pretty great:

compose well

Streams can pipe to eachother. The grouper can be implemented as multiple components, each component being a separate stream.

support backpressure

Although the underlying databases (and node-anydb) don't really support backpressure, this may change in the future.

Edit: this already did change, and node-anydb as well as most of the databases already support streams.

lower memory usage

Streams will lower the memory usage of grouped joins - intermediate row results (which contain a lot of duplication) will not be stored in memory before processing.