Support for working with multiple solutions

[razzmatazz]

At this moment there is no way to work with multiple solutions at the same time.

We need to be able to start multiple processes and properly send ops to the corresponding server processes based on buffer filename.

There is a temporary solution (that does stop-server-and-start-on-another-sln) in https://github.com/OmniSharp/omnisharp-emacs/issues/163