fix(ui): Fix performance of AllRemoteEvents::(in|de)crement_all_timeline_item_index_after

[Hywan]

This patch fixes the performance of
AllRemoteEvents::increment_all_timeline_item_index_after and
decrment_all_timeline_item_index_after.

It appears that the code was previously iterating over all items. This
is a waste of time. This patch updates the code to iterate over all
items in reverse order:

• if new_timeline_item_index is 0, we need to shift all items anyways,
  so all items must be traversed, the iterator direction doesn't matter,
• otherwise, it's unlikely we want to traverse all items: the item has
  been either inserted or pushed back, so there is no need to traverse
  the first items; we can also break the iteration as soon as all
  timeline item index after new_timeline_item_index has been updated.

I'm testing this patch with the test_lazy_back_pagination test in
#4594. With 10_000
events in the sync, the ObservableItems::push_back method (that uses
AllRemoteEvents::increment_all_timeline_item_index_after) was taking
7% of the whole execution time. With this patch, it takes 0.1%.

The call tree of TimelineEventHandler::add_item and its flamegraph
before this patch…

… and after this patch:

---

• This patch is based on fix(ui): Fix performance of TimelineEventHandler::deduplicate_local_timeline_item #4601. This patch must be merged after.
• Address [meta] EventCache storage #3280.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.75%. Comparing base (33a2cc3) to head (06d56b6).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4608   +/-   ##
=======================================
  Coverage   85.74%   85.75%           
=======================================
  Files         291      291           
  Lines       33292    33292           
=======================================
+ Hits        28545    28548    +3     
+ Misses       4747     4744    -3     

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