[Network] Prune / disconnect TCP connections gracefully

[Akkadius]

Description

This fixes an issue with our server TCP connections not closing gracefully. In the past we've used empty packet keepalives inter-process to have the server prune the connections immediately.

This PR should also significantly reduce keepalive load / chatter with servers with large amounts of processes.

Thanks to solar from Tak for code pairing on related issue #4573

Type of change

• Bug fix (non-breaking change which fixes an issue)

Testing

This is after killing 3 zone processes and below we see World disconnect the zoneserver on UV_EOF - we would see similarly for any other Server -> Server connection or Server -> Client that uses this connection class (Console, Websocket)

eqemu@83d7385f74ee:~/server$ pkill zone
  Zone |    Info    | Shutdown Shutting down... 
  Zone |    Info    | Shutdown Shutting down... 
  Zone |    Info    | Shutdown Shutting down... 
eqemu@83d7385f74ee:~/server$   Zone |    Info    | main Proper zone shutdown complete. 
  Zone |    Info    | main Proper zone shutdown complete. 
  Zone |    Info    | main Proper zone shutdown complete. 
 World |    Info    | operator() UV_EOF - Closing connection 
 World |    Info    | operator() Removed Zone Server connection from [85e3a0f1-4e45-4ae8-9dd6-69e7b8a83546] total zone_count [2] 
 World |    Info    | operator() UV_EOF - Closing connection 
 World |    Info    | operator() Removed Zone Server connection from [6146795f-f0b9-4e56-93a1-8c6535cc75b7] total zone_count [1] 
 World |    Info    | operator() UV_EOF - Closing connection 
 World |    Info    | operator() Removed Zone Server connection from [71d4c857-36a2-44e5-b8ad-24c0dca6b7cb] total zone_count [0] 

Inverse where we kill World and all zones close their connection to World

eqemu@83d7385f74ee:~/server$ pkill world
 World |    Info    | CatchSignal Caught signal [15] 
eqemu@83d7385f74ee:~/server$  World |    Info    | main World main loop completed 
 World |    Info    | main Shutting down zone connections (if any) 
 World |    Info    | main Zone (TCP) listener stopped 
 World |    Info    | main Signaling HTTP service to stop 
  Zone |    Info    | operator() UV_EOF - Closing connection 
  Zone |    Info    | operator() UV_EOF - Closing connection 
  Zone |    Info    | operator() UV_EOF - Closing connection 

Similar test with UCS

eqemu@83d7385f74ee:~/code/build$ pkill ucs
   UCS |    Info    | CatchSignal Caught signal [15] 
eqemu@83d7385f74ee:~/code/build$    UCS |    Info    | Shutdown Shutting down... 
 World |    Info    | operator() UV_EOF - Closing connection 
 World |    Info    | operator() Connection lost from UCS Server [f0c765cc-aa85-4162-8156-58c731e4809b] 
 World |    Info    | operator() Removing currently active UCS connection 

Checklist

• I have tested my changes
• I have performed a self-review of my code. Ensuring variables, functions and methods are named in a human-readable way, comments are added only where naming of variables, functions and methods can't give enough context.
• I own the changes of my code and take responsibility for the potential issues that occur