Change "dropped events" warning to debug-level log message
Change "dropped events" warning to debug-level log message

In a project where I use go-events, quite a few unit tests and
benchmarks read from a queue until they find what they're looking for,
and then stop reading. There may be more events published to the queue
afterwards, but there's no non-racy way to consume all of them and still
have the test terminate. Thus, these tests and benchmarks tend to log
many `WARN[0004] eventqueue: dropped event` messages.

Change this to the debug log level, so applications and unit tests which
use go-events are not forced to always consume all events to suppress
these messages.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

docker/go-events