Use nc for healthcheck

[penguincoder]

This is an alternative healthcheck for memcached. The prior one worked when executed with bash -c 'healthcheck' but failed silently when I removed the bash wrapper. This changes the healthcheck to use echo | nc, and I have tested this locally by taking the generated process-compose.yaml and modifying the command in the YAML. I have also tested using both 127.0.0.1 and ::1 for the listen parameter. I was not as rigorous as I should have been after I modified the last memcached PR from feedback.

[volker-schukai]

If memcache is fed with stat,

echo stats | ${pkgs.netcat}/bin/nc ${cfg.bind} ${toString cfg.port} > /dev/null 2>&1

the connection will remain open and if process-compose is used,
memcache will be terminated due to missing healthcheck.

memchache should be terminated with a quit:

echo -e "stats\nquit" | ${pkgs.netcat}/bin/nc 127.0.0.1 11211 > /dev/null 2>&1