Memory leak in Hot Rod client tests

Even though the HotRod client tests stop their servers, each test instance keeps the reference to the servers it has started (both tests extending HotRodSingleNodeTest and those extending HotRodMultiNodeTest).

In some tests, like the remote query tests, those servers' worker threads also keep references to huge thread-local io.netty.buffer.PoolThreadCache s (see attachment). Because of this, I'm sometimes seeing an OOM in the HotRod client suite. We should either clear the server references in the tests, or clear the worker threads when shutting down the server.