Simplify stream availability handling

Just have a simply number of bytes argument to avoid a lot of
complexity.

common/rdr/BufferedInStream.h

@@ -38,7 +38,7 @@ namespace rdr {
   private:
     virtual bool fillBuffer(size_t maxSize, bool wait) = 0;
 
-    virtual size_t overrun(size_t itemSize, size_t nItems, bool wait);
+    virtual bool overrun(size_t needed, bool wait);
 
   private:
     size_t bufSize;