The existing logic of only updating the fee if it is not within 2x of
the current 2-block-eta does not work well for the current mempool.
The current mempool looks a bit weird: you need ~20 sat/vbyte to even get into it,
but there is only around 5 MB of txs paying >25 sat/vbyte.
The estimates look like this:
```
>>> config.fee_estimates
{144: 25764, 25: 27075, 10: 34538, 5: 34538, 2: 34538}
```
This commit changes the logic so that we send update_fee if the old rate is
- below 75% of the current 2-block-eta (instead of 50%), or
- below the 25-block-eta
e6a0455ced