RFR: 1565: Only poll for updated MRs from GitLab

[Erik Joelsson]

This is another rather big patch, but again, with some explanation, it shouldn't be too scary. The most important part is the new `PullRequestPoller` class. This class implements a polling mechanism for PullRequests that tries to only return new or updated pull requests to a bot and guaranteeing that none are missed. The goal is to minimize unnecessary evaluations that currently take place, especially when polling Gitlab. I have added quite a few tests to verify (almost) all the functionality of the poller. For a more detailed explanation of all the challenges this poller faces, see the bug description.

In order to reduce impact, I'm only making one existing bot use the new poller, the CSRPullRequestBot. I picked this one as it should have pretty low impact overall if something goes wrong. I will followup with further conversions of other bots. I'm also touching the `CSRIssueBot` to introduce the same kind of error handling with retry that the new `PullRequestPoller` implements. I hadn't realized before that this error handling was missing in the CSRBots, and to be able to implement it properly for the `CSRPullRequestBot`, I had to also add the support for the issue polling `CSRIssueBot`.

There are features of the poller that aren't in use yet. I tried to look through all future users to make sure I could cover all existing usecases from the start.

Most of the rest of the changes are just adding `equals` and `hashCode` to a bunch of classes that we now need to compare (to know if a PR has been updated or not). This includes the `GitHubPullrequest`, `GitLabMergeRequest` and all the JSON types. For test code this also includes the `TestPullRequest` and `TestIssue` and their related data classes.

In order to test this properly, I needed to implement a way to for the `TestHostedRepository` to return copies of `TestPullRequest` instances from queries. This better simulates how new instances are always created when `PullRequest`s are queried from a remote `Forge`. The existing bot tests don't need it, but without this, it's not possible to verify the behavior of the poller. I tried a lot of different ways of implementing clone/copy (Object.clone(), copy constructor and specialized copy methods), but what I have here is what I think worked best in the end, with the least impact on existing code.

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Commit messages:
 - SKARA-1565

Changes: https://git.openjdk.org/skara/pull/1369/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1369&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1565
  Stats: 1167 lines in 25 files changed: 1102 ins; 36 del; 29 mod
  Patch: https://git.openjdk.org/skara/pull/1369.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1369/head:pull/1369

PR: https://git.openjdk.org/skara/pull/1369