Relay transactions with exponential decaying probability

[cpacia]

If we can relay txs we can increase our privacy. However, we can't validate txs so we don't know if we're relaying invalid txs or not. Bchd nodes do not currently ban for relaying invalid txs, however we open the possibility of an amplification attack if we relay everything.

To prevent this we relay with an exponentially decaying probability that resets every block.

If we ever implement banning in bchd for invalid txs we should allow a minimum number of invalid txs before a ban with the minimum set well above the expected number of relayed txs from neutrino nodes.

[zquestz]

Is there any way we can try to validate these transactions first? Seems dangerous to relay transactions where we don't know the validity. Or maybe we offer it as an option in the settings so that this is not the default behavior and users must opt in?

[cpacia]

There's no way to validate it. That's why this PR limits the number of txs relayed. But if we just used a fixed number of txs then it would be predictable. By using a decaying probability it retains plausible deniability while still limiting it.

[zquestz]

What do you think about the toggle idea? Just in case people see bad behavior from it being enabled?