Revert "Upgrade Netty to 3.6.1 and don't release the dispatcher. See #2956"

This reverts commit 800aa5286f0d91270a2ef269a3833d196d5bce26.

The commit above was violating binary compatibility by requiring a newer
Netty version which is incompatible.
Merge pull request #1203 from akka/wip-revert-2956

Revert "Upgrade Netty to 3.6.1 and don't release the dispatcher. See #2956