Improve connection management for ClusterConnectionPool

[jerroydmoore]

I noticed make test was failing for tests/test_multiprocessing_cluster.py::TestMultiprocessing::test_pool[1]. This fixes it.

• ClusterConnectionPool.make_connection: free available connections in other nodes in order to allocate a new connection.
• ClusterConnectionPool.disconnect: dereference disconnected connections in order for new connections to new nodes to be established.
• ClusterConnectionPool.count_all_num_connections: refactor to count via available connections list and in-use connections list.