Fix #1 - Introduce 4 new keep-alive related configurations to support socket keep-alive when Rserve deployed behind proxy/firewall/load-balancer

- keep-alive = when set to enabled, socket option will be set with
system wide settings for Time, Interval & Probes.
- keep-alive-time - time in seconds to override the system wide value
for this process
- keep-alive-interval - retry interval in seconds to override the system
wide value for this process
- keep-alive-probes - number of probes to override the system wide value
for this process