No more networkmanager

For varying definitions of the word "safe". :)  On user switch, `SFRestAPI` will clean itself up by canceling all outstanding requests and resetting back to a neutral state, at which point the next user will be configured on the `sharedInstance`.  So the idea on the iOS side is that there's only ever one active user context, and `[SFRestAPI sharedInstance]` should always point to it.