Add a `next_outbound_htlc_minimum_msat` to `ChannelDetails`

[TheBlueMatt]

Similar to #1126 but the opposite direction - when we hit the dust exposure threshold we may be unable to send more dust HTLCs on a channel but can happily send larger value HTLCs. To feed this info to the router we should add an htlc min field to channeldetails, and make sure the router logs appropriately (or even gives a different error message!) when we cant find a route due to hitting this.

[TheBlueMatt]

This is a pretty important issue, so hoping we get something in for 116, but not sure we're gonna make it.

[TheBlueMatt]

Much better description of this issue at #2273 (comment) (though this issue is the "actionable" version of it).