This is the first step in refactoring Worklist to allow arbitrary
number of local worklists with private segments:
- Introduce MarkingWorklistImpl<> which will eventually replace
  (and will be renamed to) Worklist.
- MarkingWorklistImpl<> owns the global pool of segments but does not
  keep track of private segments.
- MarkingWorklistImpl<>::Local owns private segments and can be
  constructed dynamically on background threads.
- Rename the existing MarkingWorklistsHolder to MarkingWorklists.
- Rename the existing MarkingWorklists to MarkingWorklists::Local.
- Rename the existing marking_workists_holder to marking_worklists.
- Rename the existing marking_worklists to local_marking_worklists.

Design doc: https://bit.ly/2XMtjLi
Bug: v8:10315

Change-Id: I9da34883ad34f4572fccd40c51e51eaf50c617bc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2343330Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69330}