Search backend: create batchingStream

Created by: camdencheek

This creates a new stream type: batchingStream. This is a stream type that batches events up to a maximum delay, then sends them to its parent stream. What's the point? Well, right now, we have a "batchEvents" function that batches events sent down a channel, but it doesn't conform to our standard streaming.Sender interface, so it can only be used at the streaming search API layer. As a bonus, this is faster than using goroutines and channels. Approximately 3-5x faster based on my benchmarks. And that's not taking into account that for streaming, we're sending events down an unbuffered channel, which will slow that down even more.