add timestamp to handshake

[billiegoose]

Supercedes #167 . It's better because:

• it invalidates old streams without requiring any old state, meaning if the program crashes and restarts, it'll invalidate any old streams as long as timestamp is monotonically increasing ¹ ²
• it's also simpler and requires less book-keeping code in hyperswarm
• a timestamp is shorter than a noise stream id
• a timestamp is the same length as a udx stream id, but doesn't suffer from relays changing the udx stream id

¹ Date.now() is not guaranteed to be monotonically increasing, but since fast reconnects are not mission critical, it should suffice, since it'll succeed "all" the time for practical purposes.

² According to MDN, performance.timeOrigin + performance.now() is guaranteed to be monotonically increasing. But I don't think the performance global is available in bare JS environments.

[billiegoose]

this default of -1 makes the timestamp behavior opt in... which may be a pain-in-the-butt for most users, who'll not benefit from this change until they modify their code to pass { timestamp: Date.now() } in the options to connect.

However, I tend to be very conservative when it comes to breaking changes, and my philosophy is usually to make all enhancements "opt in" until the next major version bump, at which time I update the defaults.

[mafintosh]

just use 0 as the default, then the uint can be unsigned, and 0 is pretty appropriate for that. also means we can just do || 0

[billiegoose]

Closed in favor of holepunchto/hyperswarm#178