Don't share WorldMapCollection. Fixes BUKKIT-4871

WorldMapCollection stores scoreboard, map (item), structure, and
village information. Scoreboards are explicitly handled globally,
while villages and structures are erroneously shared.

This commit separates the WorldMapCollections to not be shared among
custom worlds. Maps are special-cased to maintain the previous shared
behavior.
Ensure maps are stored only in the main world. Fixes BUKKIT-4827

This fixes an issue brought up by
e3613a9a616a00b4800dcf30ceff81ff71bb7025 which causes a de-sync in map
counts, which in turn causes existing maps to be re-written.