This also moves the conversion from and to legacy material to the method
calls of legacy plugins, and no longer allows them directly in the
server.
This has the side effect of fixing some legacy plugin issues, such as
SPIGOT-4288, SPIGOT-6161. Also fixes legacy items sometimes not stacking
in inventory when using addItem, a client disconnect when using legacy
items in recipes and probably some more.
By: DerFrZocker <derrieple@gmail.com>