In some cases it can happen that the http server responds with a
non-successful status code without reading the response body. In this
case curl may decide not to read from the cache since there is really no
point in sending data to the server. In case some other thread of shaka
has already called HttpFile::Flush it may end up deadlocked there
waiting for the cache to either close or become empty. Thus, we close
the cache when leaving the main thread as no data will be read by curl
after it has finished anyways.
Closes#1347
6acdcc394a