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-4872
Updated version to 1.6.4-R2.0 in pom.xml for RB.
Updated version to 1.6.4-R2.1-SNAPSHOT for development towards next release.
Merge remote-tracking branch 'upstream/master' into slow-hopper-world