Audit DisableNetwork, we_are_hibernating usage

I think a lot of our DisableNetwork checks should instead check net_is_disabled, since so much of what we're doing turning off when the network is disabled is also something we're trying to turn off when we're hibernating.

And probably some of our DisableNetwork checks should call should_delay_dir_fetches or something similar, if they're related to fetching non-bridge-descriptor directory stuff.

Possibly there should be a better designed hierarchy here.

Possibly, most of the fixes here should wait for 0.2.6, since this code is tricky.