lightning/bitcoind: adapt and batch fees estimations

This adapts our fee estimations requests to the Bitcoin backend to the
new semantic, and batch the requests.

This makes our request for fees much simpler, and leaves some more
flexibility for a plugin to do something smart (it could still lie before
but now it's explicit, at least.) as we don't explicitly request
estimation for a specific mode and a target.

Changelog-Changed: We now batch the requests for fee estimation to our Bitcoin backend.
Changelog-Changed: We now get more fine-grained fee estimation from our Bitcoin backend.

tests/plugins/bitcoin/part1.py

@@ -11,7 +11,7 @@ from pyln.client import Plugin
 plugin = Plugin()
 
 
-@plugin.method("getfeerate")
+@plugin.method("estimatefees")
 def getfeerate(plugin, **kwargs):
     time.sleep(1)
     return {}