add event rate limiter

[dustin-decker]

Add a configurable rate limiter for events.
Per-source at minimum, so we get some of every type, and maybe overall rate limiter too?

[dustin-decker]

For Capsule8 API stuff it's configurable via a ThrottleModifier when subscribing to events:
(use local godocs)
http://localhost:6060/pkg/github.com/capsule8/capsule8/api/v0/#ThrottleModifier