Fixed vanilla worlds not being readded to worlds list after unloads.

Fixes #1134, #1221, #1250, #1333

Currently we manage 2 world lists, worldServers for Forge and worlds for Bukkit.
After unloading a vanilla dimension such as the End, the world would be
removed from Forge/Bukkit lists but would only get added back into Forge's
worldServers. To fix this and prevent this from happening again in future, I
have moved all world adds for Bukkit's worlds list to DimensionManager's
setWorld method.

The world leak message config option has been removed from bukkit.yml since
Forge fixed the constant spam in latest builds. It is also important to know
if a leak occurs.