tx submission tracing details at default log level

[dcoutts]

For the node we have these tx submission tracing needs
IntersectMBO/cardano-node#527

We need to ensure the same for the proxy so we can reliably trace txs across mixed clusters.

This may be just a matter of tweaking which tracers are on/off.

[dcoutts]

So it looks like these ought to be included at the "Informational" severity level, not debug. Our default min severity level for the node is probably going to be set to Informational.