Implement connection padding (link protocol 5)

Tor has a link padding mechanism to avoid some kinds of flow-based traffic analysis. We should probably implement it some time, but it isn't as critical as some of the other stuff.

Likely subtasks:

• Reschedulable timed-event API (may be shared somewhat with #180 (closed))
• Track time of last send activity on each channel (est 2h)
• Ability to launch a task to run the padding algorithm on a live channel. (6h)
• Launch padding algorithm on certain live channels, according to padding-spec rules. (4h)
• Padding negotiation; reduced padding negotiation. (8h)
• Pad by default when link protocol >= 5 is in use. (2h)
• Implement all network parameters in padding-spec section 2; pass them around as appropriate. (4h)
• Some way to make sure padding is turned off when very dormant? (6h)
• Discuss and clarify any ambiguities we find in padding-spec section 2. (8h)