Server: Calculate maximal total block sends dynamically (#6393)

The block sends per client is 1/2 when reaching the maximal player count.

src/serverenvironment.h

@@ -342,6 +342,7 @@ public:
 
 	RemotePlayer *getPlayer(const u16 peer_id);
 	RemotePlayer *getPlayer(const char* name);
+	u32 getPlayerCount() const { return m_players.size(); }
 
 	static bool migratePlayersDatabase(const GameParams &game_params,
 			const Settings &cmd_args);