RFR: 1575: Adjust the pull request APIs in HostedRepository

[Guoxiong Li]

Hi all,

This patch mainly has the following changes in `HostedRepository` and its sub-classes. And the usages of the changed methods are also changed.

1. Change the name of the `pullRequests(ZonedDateTime updatedAfter)` to `pullRequestsAfter` so that it is consistent as the `openPullRequestsAfter`.
2. Add the new api `openPullRequests` to get all the open pull requests.
3. Change the implementation of the method `pullRequests()` to get all the pull requests (included open and closed)

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - SKARA-1575

Changes: https://git.openjdk.org/skara/pull/1367/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1367&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1575
  Stats: 99 lines in 22 files changed: 39 ins; 0 del; 60 mod
  Patch: https://git.openjdk.org/skara/pull/1367.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1367/head:pull/1367

PR: https://git.openjdk.org/skara/pull/1367