[crac] RFR: Reseed NativePRNG on checkpoint restore

[Alexey Bakhtin]

NativePRNG should be re-seeded during checkpoint/restore because it uses SHA1PRNG secure random for additional seed. It is seeded at initialization, so it is not re-seeded automatically during checkpoint/restore
Also, the internal buffer should be cleared at the checkpoint.

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Commit messages:
 - Reseed NativePRNG on checkpoint restore

Changes: https://git.openjdk.java.net/crac/pull/9/files
 Webrev: https://webrevs.openjdk.java.net/?repo=crac&pr=9&range=00
  Stats: 244 lines in 3 files changed: 242 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/crac/pull/9.diff
  Fetch: git fetch https://git.openjdk.java.net/crac pull/9/head:pull/9

PR: https://git.openjdk.java.net/crac/pull/9