FTR: Frag Table: Player Connection Duration

[proof88]

Basically this feature consists of 2 tasks:

• record timestamp of a player connected to server;
• visualize in on frag table.

1st item as I remember is needed for corner case, when a player shoots, disconnects, another player connects, with same id "accidentally" as coming from GNS, bullet hits, and we mistakenly register the frag for the newly registered player.
It is also needed for fixing this bug: #323

The bullets are already timestamped if I remember, we just need to compare the timestamps to fix the corner case.