Previously we were checking if the `hook.py` file existed for the given
filetype. ycmd has an endpoint for checking if given a filetype a
semantic completer is available. To avoid redundant requests we cache
those requests for every filetype. A semantic engine cannot be added
*after* the ycmd server is started so to avoid redundant requests we
cache those requests for every filetype and we clear the cache at server
setup, in this way if we issue a `YcmRestartServer` command the server
will be setup again and if a semantic completer is available we can use
it. Should fix#1284.
