RFR: Allow a HostedRepositoryPool to refresh the seed from upstream

[Robin Westberg]

Add support for periodically refreshing the seed repository used with the HostedRepositoryPool remote repository cache.
Also allow the refresh to fail (and instead just use the latest cache) which can be used for operations that can
tolerate a delayed update (for example, determining the project name from the jcheck configuration file which rarely
changes).

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Commit messages:
 - Allow a HostedRepositoryPool to refresh the seed from upstream

Changes: https://git.openjdk.java.net/skara/pull/874/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=874&range=00
  Stats: 110 lines in 5 files changed: 41 ins; 42 del; 27 mod
  Patch: https://git.openjdk.java.net/skara/pull/874.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/874/head:pull/874

PR: https://git.openjdk.java.net/skara/pull/874