8150607: Clean up CompactHashtable
Summary: refactored code, and added test cases for serviceability agent
Reviewed-by: jiangli, ccheung

8151368: SA: Unexpected ArithmeticException in CompactHashTable
Summary: Add missed null check
Reviewed-by: iklam, dsamersoff
Contributed-by: kubota.yuji@gmail.com

8067194: Restructure hotspot/agent/src to conform the modular source layout
Summary: Move sources under jdk.hotspot.agent
Reviewed-by: ihse, erikj, jbachorik