RFR: 8171238: Unify cleanup code used in G1 Remark and Full GC marking

[Stefan Johansson]

Hi,

Please review this re-factoring for:
https://bugs.openjdk.java.net/browse/JDK-8171238

Webrev:
http://cr.openjdk.java.net/~sjohanss/8171238/hotspot.00/

Summary:
Some small parts of the code in the G1 Mark Sweep Full GC is parallel, 
one of those is the string and symbol table cleaning as well as the 
string deduplication cleanup. They are currently run as two separate 
tasks which means the worker threads have to be started and stopped two 
times. The two tasks can be joined together and with the cleanup in 
place the Remark phase of the concurrent cycle can also be unified to 
use the same code paths.

Testing:
* JPRT
* RBT tier2 + tier3

Thanks,
Stefan