[FLINK-930] Netty Initialization is sometimes very slow

This is [FLINK-930](https://issues.apache.org/jira/browse/FLINK-930).

The issue with the Netty warning was reported in netty/netty#2412 and has been fixed in `4.0.19.Final`. I couldn't reproduce the reported problem (with and without the changes in this PR).

This PR contains the following changes:
1. Update Netty version from `4.0.19.Final` to `4.0.20.Final` and
2. Introduce `ExecutionMode` for TaskManagers, where in `LOCAL` mode the `NetworkConnectionManager` is not started (e.g. no ServerSocket binding etc.)

The 2. change speeds up the startup and shutdown time of the TaskManagers noticeably and solves/circumvents the reported problem with each test taking ~ 3 seconds for startup/shutdown.