Add the ability to create more than one ZPageServer per VM

[jkwatson]

If a user is manually instrumenting their code, and doesn't use the global singletons, they should still have a way to use the ZPages implementation. In order to do this, the ZPageServer cannot be a singleton, but must be able to be created and configured multiple times per VM.

[jack-berg]

This was solved in #4935.