Remove generics from sal.connect.api

We have a ton of generics in these APIs, coming from the time when we have had multiple specializations, perhaps (but I will not dig into long-past history).

The core point is:

We support only NETCONF and therefore we should specialize for that.

At some point netconf.git hosts both NETCONF and RESTCONF, so we should have a common API for both, but RESTCONF also has different lifecycle. Once we get around to servicing both protocols:

• the API may become slightly different
• they definitely live a different Java package
• they may even live in mdsal.git

So let's stop play-pretend and specialize the generics to their only user – which means PREF means NetconfSessionPreferences, M means NetconfMessage, etc.