A new option in server commands should show session time metrics

A new option, --time, for the 'nxserver --list' and 'nxserver --history' commands should allow administrators to visualize information about the session life.

In particular, the following information about the session should be displayed:

Created on:  <creation date>

Uptime:  <time elapsed, in seconds, since the creation time>

Disconnected on: <disconnection date>

Disconnection time: < time elapsed, in seconds, since the disconnection time>

Reconnected on: <last reconnection date>

Reconnection time: <time elapsed, in seconds, since the last reconnection time>

Terminated on: <session termination date>