feat: display unconfirmed transaction ETA

[chaserene]

this is a proposal with a future in mind where Monero may experience a long-sustained high transaction volume.

barring disruptions (like wallets not picking the right fee level, nodes having a wrong view of the chain or the txpool, or miners acting irrationally), it should be possible to give a close estimate of the inclusion time for unconfirmed transactions based on the actual values of the scaling variables (penalty-free block size, etc.), the actual content of the txpool and the transaction’s fee level.

the arrival of a new block being a Poisson point process, you can multiply the number of estimated blocks to wait by the target block time to arrive at an ETA in minutes/hours. this is useful because it’s understandable without any technical background.

the idea was inspired by Electrum, which displays something related but less useful (distance in megabytes from the tip of the next expected block) in the date column for unconfirmed transactions. similarly, I think this information is more useful than what Feather currently puts there (which is the time the transaction was first seen in the txpool).