libtorc-core: TorcNetworkRequest fixes

- fix buffer corruption when the internal buffer wraps
- better timeout handling due to interaction between read timer and
write timer in Read
- work around Qt problem with setReadBufferSize
- fix handling of completion by differentiating between completion at
the TorcNetworkRequest level and the QNetworkRequest level
- sundry cleanup
libtorc-core: Ignore any cache for stream downloads

- not actually relevant yet, as there is no cache in place