Throw exception for disabled plugin tasks. Fixes BUKKIT-3951

Without this check, any non-null reference to a plugin is considered
'valid' for registering a task in the scheduler. This is obviously
unintentional behavior and has been changed to throw an
IllegalPluginAccessException. It is now consistent with the
SimplePluginManager event registration contract.

This in affect also addresses BUKKIT-3950 for uninitialized plugin
references (ones without a description).
Correct slot types for brewing stand. Fixes BUKKIT-3937
Don't update physics when block place is cancelled. Fixes BUKKIT-3939

When a block placement happens we currently update physics on the
attempted placement and update again if the placement is cancelled.
To correct the first one we simply set the block without applying
physics. To correct the second we have to add a new method to
BlockState that lets us update without applying physics and use
this method method when putting the block back.