[FLINK-998] Close network connections when idle

1. Close idle TCP connections

Tasks enqueue envelopes to be dispatched via NettyConnectionManager. The
first task to enqueue an envelope triggers the establishment of a new
TCP connection to the target task manager. Usually, there are multiple
connections between the same task managers in order to avoid deadlocks.

This commit addresses the following problem: idle connections were never
closed. This meant that the number of TCP connections was monotonically
increasing over time. Idling connections are now closed after a
configured amount of time, see 'taskmanager.net.closeAfterIdleForMs'.

2. Added unit tests for OutboundConnectionQueue and adapted existing
tests to test concurrent enqueue and close

3. Removed unused arguments to NettyConnectionManager (low and high
water mark)