Move AvailServer AvailRuntime to Session

[richATavail]

Presently, the AvailServer holds on to one shared AvailRuntime. The result is that each connected AvailServerChannel Command channel (User - Session) shares the single runtime. Provide an option to have each Session have its own AvailRuntime.

Each AvailServerChannel command channel has a Session. Each child AvailServerChannel (non-command channel) spawned by the parent command channel shares access to the parent's Session.

This ticket requires the following:

1. Create AvailServer configurable option that allows for either, a) preserving a single shared AvailRuntime(present state) or b) allow for each Session to hold its own AvailRuntime
2. Create AvailRuntime on Session that will either be a reference to the shared AvailServer AvailRuntime (a) or have its own AvailRuntime (b) so there is one per Session.
3. Allow AvailRuntime to be shared between sessions via a token (b)
4. AvailServer jar flag indicates to optionally run using environment variable AVAIL_ROOTS to pre-populate AvailRuntime with predefined ModuleRoots.
5. Add AvailServer API feature that allows client to add/remove/create a ModuleRoot.

[richATavail]

To generalize this, we could introduce an abstract AvailServer layer with the handling of the AvailRuntime/ AvailBuilder left to the concrete implementation. The Session will hold on to a reference to both an AvailRuntime & AvailBuilder handed to it by the AvailServer. Depending on the AvailServer implementation it will either be a reference to the single shared runtime or a new one.

[richATavail]

With different AvailRuntimes and generalizing ModuleRoot source origin, #194, I'm not sure what we will do about the Repositorys (build artifacts). This is a level of complexity not yet discussed and probably should be ironed out before either task is completed.

[richATavail]

The switch to Anvil development has shifted focus away from AvailServer development. AvailServer development has been stagnant for sometime. In the event AvailServer is revisited, we will re-establish requirements at that time.