A warning log line is emitted whenever the proxy has to use a handoff
node. Monitoring these warnings can indicate a problem within your
cluster; however, you can disable these log lines by setting the
proxy conf's log_handoffs to false.
While working on this, I also noticed why many proxy log lines did
not have txn_id and client_ip -- subcoroutines. Now the logger thread
locals are copied to the subcoroutines.
Change-Id: Ibac086e1b985f566c068d083620287509de35da8
a1a4d35362