feat(Core/Optimization): Correctly document sendBuffer size and optimize it. (#18647)

* Fix comment documenting WorldSocket

The buffer is constructed with 4096 bytes but later resized in
WorldSocketThread::SocketAdded() according to the configuration
setting Network.OutUBuff (currently 65536 bytes)

* Reuse calculated packet size

Instead of recalculating the current packet size three times at
worst, calculate it once and reuse it when required.

* Reduce reserved buffer size per WorldSocket

Don't reserve 64kB of memory for every WorldSocket's output
buffer.

Instead, start with a 4kB baseline for every WorldSocket and grow
the buffer size dynamically when we have single packets that do
not fit the current buffer.

src/server/apps/worldserver/worldserver.conf.dist

@@ -364,11 +364,11 @@ Network.OutKBuff = -1
 
 #
 #    Network.OutUBuff
-#        Description: Amount of memory (in bytes) reserved in the user space per connection for
-#                     output buffering.
-#         Default:    65536
+#        Description: Amount of memory (in bytes) reserved initially in the user space per
+#                     connection for output buffering.
+#         Default:    4096
 
-Network.OutUBuff = 65536
+Network.OutUBuff = 4096
 
 #
 #    Network.TcpNoDelay: