[11u] RFR: Backport JDK-8209657: Refactor filemap.hpp to simplify integration with Serviceability Agent

[Jiangli Zhou]

Please review the backport of JDK-8209657: Refactor filemap.hpp to
simplify integration with Serviceability Agent.

webrev: http://cr.openjdk.java.net/~jiangli/backport-8209657/webrev.00/

The backport applied 99% cleanly. The only manual part involved was to
apply the following diff in filemap.hpp:

 311
 312   CDSFileMapRegion* space_at(int i) {
 313     assert(i >= 0 && i < NUM_CDS_REGIONS, "invalid region");
 314     return &_header->_space[i];
 315   }
 316 };

The backport was done with the intention to downporting additional CDS
bug fixes and improvements that are depending on this one.

Best,

Jiangli