Preventing Vim thread block on file save

Syntastic would run SyntasticCheck on file save, which would unconditionally
call _latest_file_parse_request.Response() and thus block until the request
returned from the server. We don't want that, so we throw in an explicit check
for the request being ready.
Releasing Python's GIL in C++ code where possible

Without this, all requests to the server become effectively serialized.