Ensure GetAddr fingerprinting is not possible.

[hdevalence]

Cf. https://github.com/zcash/zcash/blob/128d863fb8be39ee294fda397c1ce3ba3b889cb2/src/main.cpp#L6024-L6028

[hdevalence]

The current design guarantees that remote peers cannot inject data into our address book, because the only address book exposed outside of the network crate is the one that has information on the peers we're actually connected to.

However, the downside of only having "authenticated" (in the sense that addresses are real peers we've talked to) data is that the address book contains exact timestamps on peers we connected to. To address this, we probably want to:

1. Truncate timestamps in outbound address data to a large time interval (say 30 minutes or so? but not so large that zcashd becomes unhappy);
2. Randomize the order of outbound addresses or otherwise ensure that the truncated addresses are not sent in last-seen order.