Add a minimum inbound channel size

[npslaney]

When using ldk-node to run a public facing lightning node, it's useful to be able to manage what type of channels you accept from others. A very commonly used setting is a minimum inbound channel size, so that routing peers don't open really small, essentially useless channels.

[TheBlueMatt]

Somewhat-dup of #502. Ultimately probably needs some kind of generic "dont accept channels automatically, let me do it manually" thing.