config: improve management of htlc amount lower limit

[joostjager]

The minimum htlc amount is set when a channel is opened and thereafter it isn't possible anymore to change it. The other end of the channel will never be able to advertize a htlc_minimum_msat forwarding policy below this lower bound.

Until now, the default value was 1000 msat. Users who are not aware of the importance of this minimum value when opening the channel, may inadvertently contribute to a Lightning Network that is not able to carry sub-satoshi payments. Also when accepting a channel open request, this default value is used.

In this commit, the minimum htlc amount is lowered to 1 msat. Changing the default will not force nodes to forward lower amount payments. If the forward doesn't satisfy the node's htlc_minimum_msat policy of the outgoing channel, the htlc is still canceled back. It
remains possible to set htlc_minimum_msat to any desired value and will still default to 1000 msat.

This commit improves the capability of the network to facilitate sub-satoshi (streaming) micro payments without reducing the level of control that node operators have over the htlcs they forward.

Furthermore the rpc UpdateChanPolicy is extended with the ability set the min htlc value.

[wpaulino]

Should we consider allowing this value to be modified through UpdateChannelPolicy as a follow-up?

[joostjager]

The default effect seems real:

[joostjager]

Should we consider allowing this value to be modified through UpdateChannelPolicy as a follow-up?

So this value isn't the regular min_htlc that is advertized on the network. This is the absolute minimum that a channel will ever see in its life. After opening, it cannot be changed anymore.

The benefit of setting this to anything other than 0 (or 1) msat seems very limited and it does reduce the flexibility.

Also, if node A sets the minimum on channel open, this has consequences for node B. node B won't be able to forward anything below A's min_htlc channel parameters. It won't affect A's policy towards B.

For that reason, LND nodes connected to CL advertize a min_htlc channel policy of 0. CL has no minimum by default (or maybe it isn't even possible to set it).

Adding min_htlc setting to UpdateChannelPolicy is useful, but it isn't linked to this PR as much as one might expect.

[joostjager]

@wpaulino re-requesting review because I changed significantly more to separate fwd policy from the min htlc channel parameter.

[Roasbeef]

Not clear to me why we need to clamp this...

[joostjager]

This is the change that you requested. Not to just use the remote's min htlc in our forwarding policy, but apply our default instead. I'd rather not make this change though and stay closer to what people are used to. Let me know your final opinion on it.

[Roasbeef]

Why would accepting a zero amount HTLC ever be useful? I'm not even sure we have any tests to exercise if it'll even flow as normal for a point-to-point (direct, no hops) payment.

[joostjager]

The reason for this is pretty far-fetched. It would prepare for a future where 0 sat payments are possible. It seemed safe to me, as CL is also setting this value to zero. But the direct payment case I didn't consider. Changed to 1 msat.

[Roasbeef]

Needs rebase.

[joostjager]

rebased