Use `bitcoin-cli getmempoolinfo` to determine the lower bound feerate

[cdecker]

With the mempool full, and the minrelayfee no longer being the lower bound on fees we should likely use getmempoolinfo where available to compute the lower bound instead. It's a bit ugly that this is no longer a constant, so maybe we can round up a bit, to ensure the min mempool fee doesn't creep up while we're preparing a transaction.

This should prevent us from creating transactions that are rejected outright, and thus having funds stuck and channel opens that are not going anywhere.

In addition, rebroadcasting transactions on a regular basis could allow us to catch a moment where the min mempool fee is low, but that's likely something that is better tackled in the transaction nursery by @rustyrussell.