[JENKINS-74777] Improve locking around jenkins.branch.MultiBranchProject#getProjectFactory

[basil]

Problem

See JENKINS-74777.

Solution

As described in the above ticket, use double-checked locking.

Implementation

As in this page.

Testing done

mvn clean verify

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

Is there a reason this was marked internal rather than enhancement (or even bug, judging by the linked issues)? CC @dwnusbaum

[basil]

I marked this as an internal improvement since I did not verify effectiveness in context (i.e., in the contended environment).