Log retry attempts resulting from parallel message posts

This patch just adds a couple log entries, one when the first
attempt to post messages fails due to a race condition, and the
other when the post finally succeeds. Adding transaction IDs
will improve traceability, but that's another bp. :D

Change-Id: I188dae91f425e1effb6a12551e9d9f35c9549f71
Implements: blueprint bp/production-logging
Merge "Log retry attempts resulting from parallel message posts"