processor/stream: server blocks on stream end when batch size < 10

The `processor/stream` code reads in batches of 10 events at a time:

https://github.com/elastic/apm-server/blob/17e0f7a9f1a3aa1453e5df8b7d4cf746e9d7179d/processor/stream/processor.go#L279

Once a batch is received, they are dispatched to the publisher, which transforms and sends them through the libbeat pipeline to be recorded in Elasticsearch.

By default, agents will close the stream after 10 seconds, or after it reaches a certain size (~750K). So if an agent sends fewer than 10 events, the `processor/stream` code will generally block waiting for the stream to end before it dispatches to the publisher.

We should consider adding a timeout (or context with timeout) to the `StreamReader.Read` method to avoid this.