[feat] Improve speed to calculate page breaks

[lpagliari]

Don't clone lines on each pagination cycle; instead, use their line sized (provided by https://github.com/storytouch/ep_script_line_size/pull/1) to calculate page breaks on the fly. This allows us to remove a lot of code related to "cloned lines", "helper lines", etc.

Also:

• optimize repagination: stop paginating when pagination cycle reaches a point where the existing page break is on the same spot of the calculated page break;
• use Set (instead of an object) to store lines changed on paginationLinesChanged
• create helper methods for tests:
  • waitToHaveAnyPageBreak
  • waitToHaveAnySplitPageBreak
  • waitToHaveAnyNonSplitPageBreak
  • waitToHaveNNonSplitPageBreaks
  • waitToHaveNSplitPageBreaks
• un-pend tests and fix them so they can validate pagination again:
  • basicPagination.js: adjusted and all tests passing
  • calculatingPageNumber.js: adjusted and all tests passing
  • elementBlocks.js: not adjusted
  • enablePagination.js: adjusted and all tests passing
  • lineNumberHeight.js (should we remove this suite? We don't allow users to show line numbers anymore...): not adjusted
  • otherMoreContd.js: not adjusted
  • repaginate.js: adjusted, but with tests failing
  • scroll.js: not adjusted
  • splitElements.js: adjusted, but with tests failing
• optimize tests: don't create a new pad for each test, use the same one and clean pad content before starting each test. This can be even more efficient if we use UNDO instead of cleaning the pad, but to do that we need to change the tests in a lot of places, so this is not done on this commit;
• mark failing tests with // FIXME;

Implement part of https://trello.com/c/dJp83kE0/1971.

Full list of PRs of this feature:

• [feat] Allow plugins to use web workers ep_webpack#14
• https://github.com/storytouch/ep_script_autocomp/pull/15
• https://github.com/storytouch/ep_script_line_size/pull/1
• https://github.com/storytouch/ep_script_line_size/pull/2
• [feat] Improve speed to calculate page breaks #17
• https://github.com/storytouch/ep_draganddrop/pull/8
• https://github.com/storytouch/ep_script_scene_marks/pull/73
• https://github.com/storytouch/ep_script_copy_cut_paste/pull/34
• https://github.com/storytouch/etherpad-docker/pull/59