fix(ffi): propagate the back-pagination status immediately after a subscriber asked for it

[bnjbvr]

And add more logs to the event cache computation of the reached_start value.

Might be a reason why we ran into element-hq/element-x-ios#3768: spawning the task can be deferred a bit, so EXI doesn't receive the initial value until much later, causing it to be confused as its initial value was timelineEndReached (it's been changed here, which should also be sufficient to fix the issue).

Part of #3280.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.45%. Comparing base (a0eb934) to head (c628206).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4818   +/-   ##
=======================================
  Coverage   86.45%   86.45%           
=======================================
  Files         297      297           
  Lines       34559    34565    +6     
=======================================
+ Hits        29877    29883    +6     
  Misses       4682     4682           

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[Hywan]

Looks good to me!

[andybalaam]

Looks good, thanks!