RFR: 446: Adhere to GitHub rate limits

[Robin Westberg]

Hi all,

Please review this change that ensures that rest requests using a certain account are not performed concurrently. It
also rate-limits non-GET requests to one per second (per account).

Best regards,
Robin

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Commit messages:
 - Perform rest requests using certain credentials serially, and rate-limit non-GET requests

Changes: https://git.openjdk.java.net/skara/pull/690/files
 Webrev: https://webrevs.openjdk.java.net/skara/690/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-446
  Stats: 40 lines in 1 file changed: 37 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/690.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/690/head:pull/690

PR: https://git.openjdk.java.net/skara/pull/690