Ask peers for receive rate

[evan-forbes]

Currently, peers use configured send and receive rates. If the send rate of either peer is higher than the receive rate of the other, and this much bandwidth gets hit, it will disconnect from that peer.

This is problematic because we can't actively change this value safely without every node in the network changing at the same time.

Instead of coordinating around new values, peers should simply ask the peers what their receive rates are and adjust accordingly. If the peer doesn't respond, then they can be presumed to using the default of roughly 5MiB/s.

To close this issue, we need to create a backwards compatible mechanism for asking for the peer's receive limit during the handshake, then potentially adjusting the send rate.

[evan-forbes]

actually, upon further review with @rach-id, it doesn't look like we're disconnecting from peers that exceed the receive rate

If so this isn't needed!