Fix message ordering in threads

[germain-gg]

Related to element-hq/element-web#21113
Related to element-hq/element-web#20914

• Fix threads message ordering without server side support
• Eagerly set thread instance on root event

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This PR currently has no changelog labels, so will not be included in changelogs.

Add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is plus X-Breaking-Change if it's a breaking change.

[codecov-commenter]

Codecov Report

Merging #2215 (b70a581) into develop (6bc584b) will decrease coverage by 0.00%.
The diff coverage is 41.37%.

@@             Coverage Diff             @@
##           develop    #2215      +/-   ##
===========================================
- Coverage    57.61%   57.61%   -0.01%     
===========================================
  Files           90       90              
  Lines        16266    16269       +3     
  Branches      3742     3742              
===========================================
+ Hits          9372     9373       +1     
- Misses        6894     6896       +2     

Impacted Files	Coverage Δ
src/models/room.ts	56.00% <0.00%> (ø)
src/models/thread.ts	38.01% <15.38%> (-0.32%)	⬇️
src/client.ts	34.74% <66.66%> (-0.03%)	⬇️
src/event-mapper.ts	94.11% <66.66%> (-5.89%)	⬇️
src/sync.ts	64.08% <80.00%> (ø)

[t3chguy]

LGTM other than the PR title, you may wish to rebase this to get rid of the already-merged commits