Closing the upstream on flush will effectively terminate the ongoing
curl connection. This means that we would need re-establish the
connection in order to resume writing, this is not what we want. In the
spirit of the documentation of File::Flush
```c++
/// Flush the file so that recently written data will survive an
/// application crash (but not necessarily an OS crash). For
/// instance, in LocalFile the data is flushed into the OS but not
/// necessarily to disk.
```
We will instead wait for the curl thread to finish consuming what ever
might be in the upload cache, but leave the connection open for
subsequent writes.
Fixes#1196
53d91cd0f1