Using add_definitions, workaround cmake warning

We haven't been building LLVM in-tree for many months now so we can use this now
just fine.
Vim doesn't execute extra conf preload

This is executed inside the server now, as it should be.
Adding requests-futures subrepo to third_party
Adding Requests as submodule to third_party
Event and completion request are now async

This results in a much snappier Vim.