Add wait for exit function in WakeableLooper and fix exiting code in SocketChannelHelper

[nwangtw]

Add waitForExit() function in WakeableLooper and use it to avoid racing condition in SocketChannelHelper's forceFlushWithBestEffort() function.

The main motivation of this change is the flaky unit tests in HeronServerTest. However it is possible to happen in real topologies as well in the exiting process.

In the test, a write task is scheduled and executed in a child thread and the main thread calls the flush function. Both threads are consuming the same buffer. When the main thread calls writeToChannel() when buffer size is 0, an assert(and exception) would be triggered.
public class OutgoingPacket {
...
public int writeToChannel(SocketChannel channel) {
int remaining = buffer.remaining();
assert remaining > 0;
....

[huijunw]

what happens if the looper does not stop after 10 seconds

[nwangtw]

The function stops waiting and returns false.

In this case, the code works pretty much the same way as before after the waiting. Except exitLoop() set a flag to true so the looper won't do any real work afterwards.