Support complete service registry introspection

Extra methods to allow full introspection of service registry contents (all services and all instances). `ServiceResolve`r heavily refactored to be type-safe: `ServiceAlias` and `ServicePath`, `URI`.

These changes are required to implement 4finance/micro-infra-spring#79 . Once merged and release, https://github.com/4finance/micro-deps-spring-config/commit/d7bf1e00a45290ad5f3169b1be04886f3c5d1d18 can be pull requested.
Maybe... I guess `stop()` removes dependency from ZK, but test server is closes after each test anyway.