Improve speed and memory use of FixedMetadataValue. Fixes BUKKIT-1460

FixedMetadataValue currently just extends LazyMetadataValue with a value
that never changes. While this works it is a lot of unneeded overhead
that causes FixedMetadataValue to be a lot slower and use a lot more
memory than one would expect. To correct this we store the value directly
in FixedMetadataValue and override the the appropriate methods to use it.

Ideally we would modify FixedMetadataValue to no longer extend
LazyMetadataValue as this would give a very large memory savings. However,
this is not currently done for backwards compatibility reasons.