Update service & plugin implementations to use non-nullable proxies

[bernardnormier]

C++ proxies can now be non-nullable (e.g. ObjectPrx, RouterPrx) or "nullable" (std::optional<ObjectPrx>, std::optional<RouterPrx>).

The implementation of the services is now using primarily / only nullable proxies because this was the most straightforward translation from the previous representation (shared_ptr<XxxPrx>). We want to update these implementations to use the new proxy syntax and non-null semantics where appropriate.

This update should also include replacing calls to uncheckedCast by calls to the Prx constructor.

• [ ] IceGrid
• [ ] IceStorm
• [ ] Glacier2
• [ ] IceBridge
• [ ] IceDiscovery
• [ ] IceLocatorDiscovery
• [ ] IceBox

(nothing to do for the transport plugins)