replace netcat with socat for liveness probe/metrics

[cidermole]

We've had an issue on Google Kubernetes Engine, on a node with kernel version 4.14.138+, where liveness probes would regularly fail some percentage of the time.

We've traced the problem down to the poll() system call sometimes failing in the nc command used in the liveness probe, whereupon nc returns an empty response, despite the TCP connection from Zookeeper clearly sending back an imok.

Netcat uses select(), poll(), read(), where poll() sometimes throws an error because Zookeeper has closed the TCP connection. Socat uses select(), read(), which works here.

[opcache]

yes,thank you