Facilitate running load tests inside WebHost

[bartelink]

See also #57 which contains followup work - this PR takes us to the point of a working WebApp and being able to use the CLI to drive it:

• Pull out store-neutral building logic
• Pull out test building logic (yielding good results, but pulling on the thread is taking longer than one might expect)
• Make Web host work
• Allow CLI to run against Web Host (as an other option alongside Cosmos, Es and Mem
• Make WebHost have same storage options as CLI does

[bartelink]

@voronoipotato FYI there's a copy of the samples/Store/Web on master now which compiles and functions as a web api (but no front end). From here, in theory all we need is for someone to 'just' build a nice Fable-based FE (and possibly polish the Web layer a bit more - the wiring is not idiomatic aspnetcore as I've little experience with it)

In real live, my next step on the demos/docs side of the house is probably to do a https://www.todobackend.com port based on https://github.com/ChristianAlexander/dotnetcore-todo-webapi/blob/master/src/TodoWebApi/Controllers/TodosController.cs as this gives me a frontend (and I can build a CLI-driven benchmark which meets my needs in terms of being able to optimize perf).

cc @mateusz-kula