Add heartbeat support (stop all cores in case of TCP socket issues)

Add heartbeat support (stop all cores in case of TCP socket issues)

"heartbeat timeout" (in second) can be specified as a global parameter
in PROX config file. If set, a timer is started when the first
command is received from the TCP socket. This timer is reset at each
commands received through the TCP socket. If the timer expires, then
- all cores are stopped
- the TCP socket is closed, causing an error at client side.

This feature helps in case a script starts PROX and the traffic generated
through PROX causes issues to the control plane.

Change-Id: I900f22fa091786a564f6b7d846f5abc2c5cbcc58
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>