The polling architecture with 200 ms sleep in ProcessEventMessages()
has two drawbacks:

- it imposes a rate limit of 5 events per second for any input source
- it imposes an average latency of 100 ms per event

The polling loop can be avoided using a goroutine per worker that
posts incoming events to the `messageStream` channel, and pending on
that channel in ProcessEventMessages().

A short `messageStream` queue guarantees fairness: no worker's latest
event will be postponed for more than the length of the channel.

Signed-off-by: Jeff Learman <jjlearman@gmail.com>