[crac] RFR: Support repeated checkpoint and restore operations

[Radim Vansa]

* VM option CRaCCheckpointTo is recognized when restoring the application (destination can be changed)
* The main problem for checkpoint after restore was old checkpoint image mmapped to files (CRaC-specific CRIU optimization for faster boot). Before performing checkpoint we transparently swap this with memory using anonymous mapping.

-------------

Commit messages:
 - Support repeated checkpoint and restore operations

Changes: https://git.openjdk.org/crac/pull/57/files
 Webrev: https://webrevs.openjdk.org/?repo=crac&pr=57&range=00
  Stats: 391 lines in 9 files changed: 349 ins; 15 del; 27 mod
  Patch: https://git.openjdk.org/crac/pull/57.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/57/head:pull/57

PR: https://git.openjdk.org/crac/pull/57