Fix NPE in CleanupStaticMembers.

Determine if this is really necessary. No point cleanup up after
yourself in a sewer.
Don't throw a NullPointerException if the injection fails.
Use socket as key instead of input stream.
It's an injector container.
We might revert this some other time, if we need to support other JVMs
Identify player connections by socket address.

It's the only thing that will not not be removed when a
network manager closes, making it relatively safe to block on.