KeyspaceManager is not threadsafe

[andrewswan]

As discussed on the Google group, the KeyspaceManager class is not threadsafe, by virtue of extending ManagerOperand, which uses a single class-scoped IConnection.

Making KeyspaceManager threadsafe would make it much more usable in multi-threaded environments, e.g. when listing all keyspaces currently defined in the cluster. Currently the workaround is to instantiate a new KeyspaceManager for each invocation, which is expensive.