Motivation:
I did not see any tangible advantage to the padding.
The only other field that was guarded was a rarely changed object reference to a BitSet.
Without the padding, there is also no longer any use of the inheritance hierarchy.
The padding was also using `long`, which would not necessarily prevent the JVM from fitting the aforementioned object reference in an alignment gap.
Modification:
Move all the fields into the InternalThreadLocalMap and deprecate the stuff we'd like to remove.
Result:
Simpler code.
This resolves the discussion in https://github.com/netty/netty/issues/9284
5c1613df81