AdapterIO: Increase consistency

* Moved data locking in the internal buffer class, includes handling
of backend writes.
* Takes advantage of inheritance for getting the size. The choice is
made depending on the flags. This allow the implementer to easily
return a custom size by reimplementing GetSize. At the same time
a plain BAdapterIO can still have it's total size set, but the behavior
will change depending it's mutable or not.
* Some decisions are now made by considering everything in absolute
values.
* Other minor fixes.

headers/private/media/AdapterIO.h

@@ -72,10 +72,8 @@ private:
 			int32					fFlags;
 			bigtime_t				fTimeout;
 
-			off_t					fBackPosition;
-	mutable RWLocker				fLock;
-
 			RelativePositionIO*		fBuffer;
+			off_t					fTotalSize;
 
 			BInputAdapter*			fInputAdapter;