Improvements to RefreshLinksJob/DeleteLinksJob locking

Improvements to RefreshLinksJob/DeleteLinksJob locking

* Removed the lockAndGetLatest() call which caused contention problems.
  Previously, job #2 could block on job #1 in that method, then job #1
  yields the row lock to job #2 in LinksUpdate::acquirePageLock() by
  committing, then job #1 blocks on job #2 in updateLinksTimestamp().
  This caused timeout errors. It also is not fully safe ever since
  batching and acquirePageLock() was added.
* Add an outer getScopedLockAndFlush() call to runForTitle() which
  avoids this contention (as well as contention with page edits)
  but still prevents older jobs from clobbering newer jobs. Edits
  can happen concurrently, since they will enqueue a job post-commit
  that will block on the lock.
* Use the same lock in DeleteLinksJob to edit/deletion races.

Change-Id: I9e2d1eefd7cbb3d2f333c595361d070527d6f0c5