Add start() and stop() to SystemAdmin

This patch adds start() and stop() to SystemAdmin interface. This can be useful for e.g. kafka.admin.AdminClient which needs to be started before it can be used.

Since we add this method in interface and expect AdminClient to be stateful and probably has its own thread, there will be higher cost to instantiate a new SystemAdmin. Thus we probably want to re-use the SystemAdmin instances instead of creating SystemAdmin on demand when needed. Therefore, this patch also adds SystemAdmins class to help manage a map from system to SystemAdmin, similar to the existing SystemProducers class in Samza.