Omnisharp: smarter solution file finder

When loading the Omnisharp server, YCM tries to find a suitable solution
file to feed it. Instead of giving up when finding multiple solution
files, it now tries to find one named like the edited files' folder at
the solution level, e.g. if we have bla/Project.sln and we are editing
bla/Project/Folder/Whatever/File.cs, we use bla/Project.sln.