Refresh signs on chunk load

[tpwalke2]

In #108, regenerated chunks do not have their sign configurations removed.

With this issue, update the mod as follows:

• listen to the ServerChunkEvents.CHUNK_LOAD event
• on chunk load, refresh all signs that fall in that chunk
  • this should use the existing logic to add/update/remove as required

This depends on #109 to ensure that the chunk load sign retrieval does not negatively impact server performance.