Simplify dispatch interceptors

[bernardnormier]

Dispatch interceptors (https://doc.zeroc.com/ice/3.7/server-side-features/dispatch-interceptors) correspond very roughly to middleware in IceRPC, except they also provide "replay support": Ice automatically resets the InputStream and OutputStream to allow the dispatch interceptor to retry a dispatch.

This "replay support" was added for the Freeze evictor, to allow Freeze to automatically retry a dispatch upon a deadlock exception. This is no longer a concern for Ice 3.8 since there is no Freeze.

This replay support is quite complicated because it supports multiple concurrent dispatch "sharing" the same Incoming object.

I propose to greatly simplify Dispatch interceptors by removing this "replay support". With this proposal, a dispatch interceptor can log the request and perform other processing (very limited in Ice since the request is opaque), but cannot dispatch the request more than once.