Message buffer filled log message should be an error message

[rllola]

I have encounter the case where I filled the buffer faster than I was reading it and messages were dropped. Took me a while to figure out because I didn't see any errors.

I noticed the INFO message after a while (see https://github.com/libp2p/go-libp2p-pubsub/blob/master/pubsub.go#L928). It should be made more evident to people that messages are being dropped to avoid confusion by showing ERROR or at least a WARN tag in the logs.

[lthibault]

It seems to me this is definitely not an ERROR-level event, since it does not invalidate the state of the system.