Describe the bug

Due to the logic in TTree::GetCacheAutoSize, if fAutoFlush == 0 then the size of the TTreeCache is set to 0 which in turn disables it.

However there are separate mechanisms to disable the TTreeCache if desired (e.g. TTree::SetCacheSize) and even if fAutoFlush == 0, when reading over the network, we can benefit greatly from TTreeCache's pre-fetching.

Additional context

• discussion and debugging: https://mattermost.web.cern.ch/root/pl/uzub3wwt3bdxxcdu4jmxxyrmby
• summary: https://mattermost.web.cern.ch/root/pl/xmtuq55j3pnc7cp7f8zyhf5ujw