Add GarbageCollectorMXBean for Remark and Cleanup pause time in G1

On behalf of Yude Lin <yude.lyd@alibaba-inc.com>:

Problem description:

GarbageCollectorMXBeans for G1 include:
G1 Young Generation
G1 Old Generation
None of these currently count the Remark time or Cleanup time.

jstat does include the Remark/Cleanup time in the 'CGCT' field.

In CMS (with ParNew), the beans are:
ParNew
ConcurrentMarkSweep
The Remark time counts towards the 'ConcurrentMarkSweep' bean.

This inconsistency between G1 beans and jstat, or between G1 and CMS,
might cause confusion for users.

Proposal:

Add Remark and Cleanup time to a G1 bean.

Looking at the code, intuitively, the code change should be moderate. Including
1. changing the timer in VM_G1PauseConcurrent::do_it() to a G1MonitoringScope
2. this G1MonitoringScope should use a new memory manager
3. this G1MonitoringScope should use _conc_collection_counters