lnwallet+docs: minrelayfee always above fee floor

[ellemouton]

The minimum relay fee is always ensured to be above our fee floor except
in the very first min relay fee query to bitcoind. This commit ensures
that the fee floor is respected in this first query.

Fixes #6072

[sangaman]

Thank you! I tested this branch to open a channel on my node that was previously stuck on 250 sat/kw which was insufficient for channel broadcasts on testnet.

The channel funding tx broadcast worked, but interestingly I got a higher fee rate than expected of 3 sat/byte, see below

[INF] CHFD: Performing funding tx coin selection using 747 sat/kw as fee rate

https://blockstream.info/testnet/tx/8d21ecf648fd99a7781f16ef99e90d52d120f42b7c53dcc49c38f5a00f7cc64f

This was during a flurry of blocks on testnet (multiple within a minute) and near empty mempools and 1 sat/byte recommended by mempool.space. So I found this a bit odd, but I'll try a channel open again when testnet is under more normal conditions with blocks spaced a few minutes apart.

[Crypt-iQ]

Thank you! I tested this branch to open a channel on my node that was previously stuck on 250 sat/kw which was insufficient for channel broadcasts on testnet.

The channel funding tx broadcast worked, but interestingly I got a higher fee rate than expected of 3 sat/byte, see below

[INF] CHFD: Performing funding tx coin selection using 747 sat/kw as fee rate

https://blockstream.info/testnet/tx/8d21ecf648fd99a7781f16ef99e90d52d120f42b7c53dcc49c38f5a00f7cc64f

This was during a flurry of blocks on testnet (multiple within a minute) and near empty mempools and 1 sat/byte recommended by mempool.space. So I found this a bit odd, but I'll try a channel open again when testnet is under more normal conditions with blocks spaced a few minutes apart.

747 sat/kw is roughly 3 sat/byte
oops misread comment my bad

[Roasbeef]

This was during a flurry of blocks on testnet (multiple within a minute) and near empty mempools and 1 sat/byte recommended by mempool.space.

lnd will just use w/e is returned by the fee estimator. Testnet is weird at times, and you'll see transactions over paying routinely as there're no economic constraints, so I wouldn't take too much stock in estimate returned on testnet.

[Roasbeef]

LGTM ☄️