Add better support for new ZK 3.5 admin server

[hannosch]

In 0844aa8 I've changed our internal testing hooks to explicitly configure the new admin server port, so it won't conflict with multiple local ZK servers and allows us to test against a ZK 3.5 ensemble.

But we might want to actually use the new admin server for the four letter words, for example in the client.command / server_version API's.