Added back ChunkGC disabled by default.

ChunkGC was removed since it affected mods such as BoP, IC2, and others.
If your server is experiencing lag due to mods leaking too many chunks,
then you can enable this setting. Be warned that you may experience other
issues with this turned on as Forge/Vanilla has no ChunkGC by default.
To enable ChunkGC, you need to set chunk-gc: enabled
to true in bukkit.yml. You will not see it on first startup unless you
save or add it manually.

* Fixed skip-ticks-near-unloaded-chunks check.
* Added new config option "tileentities.skip-ticks-not-near-player" which
is false by default. Enabling this option will skip TileEntity ticks when
the player is not near. Forced chunks are not affected.