[crac] RFR: JDKResource priorities

[Alexey Bakhtin]

Added priority enumeration for the JDK resources
It will allow better handling the order of checkpoint notifications for dependent resources

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Commit messages:
 - JDKResource priorities

Changes: https://git.openjdk.java.net/crac/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/?repo=crac&pr=8&range=00
  Stats: 32 lines in 6 files changed: 22 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/crac/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/crac pull/8/head:pull/8

PR: https://git.openjdk.java.net/crac/pull/8