ARROW-207: Extend BufferAllocator interface to allow decorators around BufferAllocator

[vkorukanti]

Currently AllocationManager needs BufferAllocator instance reference for accounting purposes.
Having a decorator around the BufferAllocator disturbs the accounting in AllocationManager.
Add an unwrap method to allow getting the reference to inner BufferAllocator instance and use
this method in AllocationManager to get the actual BufferAllocator.

/**

• Unwrap the class so that exposes the provided interface, if possible. Otherwise, throw Exception.
• @param c

•      The class or interface that you want this class to implement/extend.
  

• @return The instance of that class related to 'this'
  */
  T unwrap(Class c);

@StevenMPhillips: Could you please review this patch?

[StevenMPhillips]

If AllocationManager assumes the the BufferAllocator is a specific implementation class, what is the point of the interface? It seems to me we should either fix the interface or get rid of it.

[vkorukanti]

I agree, current AllocationManager depends on the implementation of BufferAllocator, BaseAllocator, than the interface.

[jacques-n]

I suggest we abandon this and come up with a different approach for the extensibility you're trying to achieve.