Make use of an array to store FastThreadLocals and so allow to also use it in PooledByteBufAllocator that is instanced by users.

Motivation:
Allow to make use of our new FastThreadLocal whereever possible

Modification:
Make use of an array to store FastThreadLocals and so allow to also use it in PooledByteBufAllocator that is instanced by users.
The maximal size of the array is configurable per system property to allow to tune it if needed. As default we use 64 entries which should be good enough.

Result:
More flexible usage of FastThreadLocal