(teststabilization) RFR: 8231506: Fix some instabilities in a few networking tests

[Daniel Fuchs]

Hi,

Please find below a patch for:

https://bugs.openjdk.java.net/browse/JDK-8231506
8231506: Fix some instabilities in a few networking tests

webrev:
http://cr.openjdk.java.net/~dfuchs/webrev_8231506/webrev.00/


There's one remaining test that still uses the wildcard
(SocketImplCombinations). It has been observed failing in the past,
and hopefully this change will fix it.

The UnreferencedMulticastSockets, which has also been observed
failing is also be updated with some more diagnosis.

Finally, the DigestEchoServer$TunnelingProxy class has also been 
observed causing some instabilities in the httpclient tests that use it: 
the proxy tunnel doesn't properly propagate errors from downstream to 
upstream, and conversely, which sometimes leaves the reader waiting for 
data that will never come. This change should fix that.

best regards,

-- daniel