This is a rather basic adaptation of techage's forceload blocks/tiles to make them work outside of techage.
Techage is originally by joe7575 (Joachim Stolberg) and licensed under the AGPLv3, see LICENSE.txt for more information.

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These forceload blocks/tiles will forceload the mapblock they are placed in, but only when the player that owns them is online.
Use the minetest.conf setting "techage_forceload_max" (default 16) to control how many of these each player is allowed to use.
Additionally, you will probably want to set the overall forceload maximum for the server to at least this number times the expected maximum number of players.