Persistent Introducer connections leak IP addresses even when idle

[cloud_trouble]

When the tahoe service is running it maintains a TCP connection to the Introducer (and storage servers?) even when idle. This can lead to unintentional information leaks.

For example, every coffee shop a user visits learns the user's home IP address (if she runs a node there), even if she hasn't used tahoe.

This might conflict with user's expectations: when a user "puts" or "gets" a file, she knows she is connecting to the tahoe nodes; but when the service is running in the background, she may not expect it to be maintaining connections.