Add producer name to trx idle log message

[matthewdarwin]

In debugging #1349, it was observed by Kevin that it would be good to log the producer name for the following message:

Block #317926278 trx idle: 237us out of 199964us, success: 3, 578us, fail: 230, 197119us, transient: 0, 0us, other: 2030us

producer_plugin.cpp:290 in leap 4.0.3

This extra information would allow creating charts per block producer on https://blockchain.stats.eosnation.io

[bhazzard]

This seems like a simple enough change, and has obvious value.