Tune Juliet networking settings

The juliet node configuration is currently way too liberal with its buffer space (i.e. the settings in the chainspec found for juliet, as well as the memory buffer configuration found on the node). As soon as the "essentially unlimited" configuration runs stable, it should be retried with more conservative settings, until a better set of values is found.
This requires the conman branch to be merged, see #4551 for details.