fix: spendable balance in lnd

[im-adithya]

Issue

It is wrongly mentioned that -60000 sats are incoming although there are no channels waiting to be closed, and also this happens because total balance is less than the spendable balance (hence the -ve sign) which can never be the case

Description

This happens because we remove the ReservedBalance from the TotalBalance in LND which makes TotalBalance less than ConfirmedBalance

From LDK docs

total_onchain_balance_sats: u64
The total balance of our on-chain wallet.
spendable_onchain_balance_sats: u64
The currently spendable balance of our on-chain wallet.
This includes any sufficiently confirmed funds, minus total_anchor_channels_reserve_sats.

That's why we remove total_anchor_channels_reserve_sats from total_onchain_balance_sats in LDK

But in LND, spendable_onchain_balance_sats doesn't remove any reserves, it gives the actual confirmed balance. So this change subtracts total_anchor_channels_reserve_sats from both

[rolznz]

utACK