Better implementation of AttributeMap

Motivation:
The old DefaultAttributeMap impl did more synchronization then needed.

Modifications:
* Rewrite DefaultAttributeMap to not use IdentityHashMap and synchronization on the map directly. The new impl uses a combination of AtomicReferenceArray and synchronization per chain (linked-list). Also access the first Attribute per bucket can be done without any synchronization at all and just uses atomic operations. This should fit for most use-cases pretty weel.

Result:
Synchronization is per linked-list and the first entry can even be added via atomic operation.