Refactor to simplify testing -- separate server from logic

[brianru]

It is currently very difficult if not impossible to test the logic of the language servers without starting a server and poking it with the server's request and notification methods. However, that makes it very difficult to isolate and test parts of the logic.

I think if we want to continue improving the functionality of each server, as opposed to just adding more servers, we need to build out a suite of unit-tests for each verifying completions and other features return what we want in a wide range of scenarios.

I would like to start by separating the language server protocol aspect from the underlying logic and state management in the servers.

What do you think @jmrog @anneeb ?

[jmrog]

This sounds right to me.