[crac] RFR: Reseed NativePRNG on checkpoint restore [v3]

[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.

Alexey Bakhtin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Prevent NativePRNG usage in the beforeCheckpoint/afterRestore
 - Merge branch 'crac' of https://github.com/openjdk/crac into NativePRNG
 - Use ReentrantReadWriteLock
 - Reseed NativePRNG on checkpoint restore

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

Changes:
  - all: https://git.openjdk.java.net/crac/pull/9/files
  - new: https://git.openjdk.java.net/crac/pull/9/files/196edc75..dd461601

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=crac&pr=9&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=crac&pr=9&range=01-02

  Stats: 274 lines in 13 files changed: 255 ins; 0 del; 19 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