Deadlocks under load

[cavaliercoder]

When under load with zabbix_agent_bench, if all zabbix_agentd: listener pids happen to process identical item keys at the same time, they appear to deadlock at futex().

I didn't find any calls to futex() in the Zabbix or g2z codebase, so I'm assuming it's occurring somewhere in Go's concurrency logic.