Reject HTLC forwarding requests if fee-bumping reserves are low

[ariard]

If the numbers of pending HTLCs is too high and our fee-bumping reserves are too low, we should reject new HTLC forwarding request to avoid a situation where we cannot cover high feerates at current level of network mempools congestion. This type of mechanism is not implemented by any Lightning implementation, afaik.

This type of checks could be introduced in the same code path where we're checking other BOLT4 checks in decode_update_add_htlc_onion() or offloaded to the LDK user if we can have a generic HTLC interception interface.

Sounds a follow-up of #2089 and #2320.

[TheBlueMatt]

This really should just be a part of the greater approach to #2320. Let's not blow up our issue count :)

[ariard]

This really should just be a part of the greater approach to #2320. Let's not blow up our issue count :)

At the same time when the issue can be implemented on its own granularly, can be its own. Yeah merging it with #2320.