6871109: G1: remove the concept of the scan only prefix
Summary: Removed scan only regions and associated code. The young portion of the collection set is now constructed incrementally - when a young region is retired as the current allocation region it is added to the collection set.
Reviewed-by: apetrusenko, iveresov, tonyp

Summary: ensure that max and committed are non-zero (currently: at least as large as the region size).
Reviewed-by: iveresov, mchung

Summary: size_t max_size() hides size_t max_size() const.
Reviewed-by: jmasa, ysr

Summary: G1 reports committed memory instead of reserved memory from the Runtime.maxMemory() method
Reviewed-by: ysr, jmasa

Summary: It introduces the necessary memory pools for G1.
Reviewed-by: mchung, ysr