This commit upgrades us to Netty 4.1.25. This upgrade is more
challenging than past upgrades, all because of a new object cleaner
thread that they have added. This thread requires an additional security
permission (set context class loader, needed to avoid leaks in certain
scenarios). Additionally, there is not a clean way to shutdown this
thread which means that the thread can fail thread leak control during
tests. As such, we have to filter this thread from thread leak control.
Jason Tedor