Remove condition in ChannelHandlerAdapter.isSharable() by caching the result of the annotation lookup.

Motivation:
Remove the synchronization bottleneck and so speed up things

Modifications:
Introduce a ThreadLocal cache that holds mappings between classes of ChannelHandlerAdapater implementations and the result of checking if the @Sharable annotation is present.
This way we only will need to do the real check one time and server the other calls via the cache. A ThreadLocal and WeakHashMap combo is used to implement the cache
as this way we can minimize the conditions while still be sure we not leak class instances in containers.

Result:
Less conditions during adding ChannelHandlerAdapter to the ChannelPipeline