Rename `Filter` to `Handler`

Now that they're used to "do everything"
rather serve the particular purpose of transforming `Message`s,
and given they're a very small interface on top of `ChannelHandler`s,
filters are named handlers like in Netty.
build.gradle: v4.0.0-ALPHA
Embrace tags in the message API

The API always expects tags to make sure
transforming handlers won't destroy tags by accident.