Clean up a number of issues regarding Netty on the 4.x branch:

1. We only test Netty when JDK == 6, but now our minimum is 8.  We should add a new variant specifically for Netty to ensure it's tested regardless of Java version
2. We should upgrade our Netty dependency to the latest patch
3. We should remove all references to the streamType connection string parameter, as it's no longer supported
4. We should rename the now-test-only system property to org.mongodb.test.async.type