Figure out how to communicate on-chain fees to the user

[tnull]

We currently have no good way of communicating what on-chain fees the user could expect, e.g., for channel opens. It's however not immediately clear to me how we want to expose to-be-expected fees without just dumping the whole complexity of fee estimation on the user.

We probably don't want to complicate the connect_open_channel interface with yet another parameter, but maybe a combination of giving some visibility into our current fee estimate cache and the capability to set a fee rate upper bound via Config might be an option here,

[tnull]

Closing as duplicate of #176