Separate events subsystem

* Events subsystem merged from `server/events.go` and
  `utils/jsonmessagepublisher.go` and moved to `events/events.go`
* Only public interface for this subsystem is engine jobs
* There is two new engine jobs - `log_event` and `subscribers_count`
* There is auxiliary function `container.LogEvent` for logging events for
  containers

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)
[solomon@docker.com: resolve merge conflicts]
Signed-off-by: Solomon Hykes <solomon@docker.com>
Rename "log_event" to "log"

Signed-off-by: Solomon Hykes <solomon@docker.com>