Refactor the sliding sync stream method

[poljar]

Tested with the sliding sync integrations tests, so hopefully nothing broke.

[gnunicorn]

is that mainly a code move or did you also change any of the internals? What prompted the change?

[codecov]

Codecov Report

Base: 72.82% // Head: 72.84% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (12d7a7f) compared to base (4115975).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1446      +/-   ##
==========================================
+ Coverage   72.82%   72.84%   +0.01%     
==========================================
  Files         121      121              
  Lines       13292    13292              
==========================================
+ Hits         9680     9682       +2     
+ Misses       3612     3610       -2     

Impacted Files	Coverage Δ
crates/matrix-sdk/src/sliding_sync.rs	0.00% <ø> (ø)
crates/matrix-sdk-crypto/src/identities/manager.rs	72.15% <0.00%> (+0.78%)	⬆️

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[poljar]

is that mainly a code move or did you also change any of the internals?

It's mainly a code move, but you can't call yield/continue/break in the sync_once() method some things got moved around a bit more then what I would like to, not having the ability to test this myself in EX.

What prompted the change?

I would like to connect the instrumentation in this method to a root span. The easiest way to achieve this is to call future.instrument(root_span). So I would like to have the bulk of the logic in a separate future.

[poljar]

What prompted the change?

This PR #1447