RFR 8046339: sun.rmi.transport.DGCAckHandler leaks memory

[Ivan Gerasimov]

Hello!

This is a request to help review a fix for the problem which can lead to 
memory leakage under some rare circumstances.
The fix itself is quite simple - it just makes sure the handler being 
released is also removed from the set of tracked handlers.

I've also augmented an existing regression test to make sure we're not 
observing the memory leak anymore.

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8046339
WEBREV: http://cr.openjdk.java.net/~igerasim/8046339/00/webrev/

Would you please help review it?

Sincerely yours,
Ivan