Add count based unload limit for mapblocks

minetest.conf.example

@@ -94,6 +94,9 @@
 #random_input = false
 #    Timeout for client to remove unused map data from memory
 #client_unload_unused_data_timeout = 600
+#    Maximum number of mapblocks for client to be kept in memory
+#    Set to -1 for unlimited amount
+#client_mapblock_limit = 1000
 #    Whether to fog out the end of the visible area
 #enable_fog = true
 #    Whether to show the client debug info (has the same effect as hitting F5)