feat(send queue): maintain ordering of sending

[bnjbvr]

Prior to this patch, the send queue would not maintain the ordering of
sending a media then a text, because it would push back a dependent
request graduating into a queued request.

The solution implemented here consists in adding a new priority column
to the send queue, defaulting to 0 for existing events, and use higher
priorities for the media uploads, so they're considered before other
requests.

A high priority is also used for aggregation events that are sent late,
so they're sent as soon as possible, before other subsequent events.

Let's say it's part of #4201.

[codecov]

Codecov Report

Attention: Patch coverage is 98.36066% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.94%. Comparing base (d614878) to head (84af9e2).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-base/src/store/traits.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4259      +/-   ##
==========================================
- Coverage   84.94%   84.94%   -0.01%     
==========================================
  Files         274      274              
  Lines       29805    29851      +46     
==========================================
+ Hits        25319    25357      +38     
- Misses       4486     4494       +8     

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[andybalaam]

This looks very nice. Some questions, nothing blocking.