lnpeer: raise chan fees using update_fee more aggressively

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

electrum/simple_config.py

@@ -24,7 +24,8 @@ from .logging import get_logger, Logger
 FEE_ETA_TARGETS = [25, 10, 5, 2]
 FEE_DEPTH_TARGETS = [10_000_000, 5_000_000, 2_000_000, 1_000_000,
                      800_000, 600_000, 400_000, 250_000, 100_000]
-FEE_LN_ETA_TARGET = 2  # note: make sure the network is asking for estimates for this target
+FEE_LN_ETA_TARGET = 2       # note: make sure the network is asking for estimates for this target
+FEE_LN_LOW_ETA_TARGET = 25  # note: make sure the network is asking for estimates for this target
 
 # satoshi per kbyte
 FEERATE_MAX_DYNAMIC = 1500000