event cache: handle blocking correctly

[bnjbvr]

This should "fix" ignoring/unignoring users, which had regressed likely since the introduction of the event cache.

This is still not perfect: in theory the sliding sync room cache should also be emptied every time we get an ignored/unignored change, but better to wait for #3092 to land, instead of trying to work around it somewhere else.

Part of #3058, should help with element-hq/element-meta#2382 too.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 82.98%. Comparing base (9ef465f) to head (9071542).

Files	Patch %	Lines
crates/matrix-sdk/src/event_cache/mod.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3378      +/-   ##
==========================================
+ Coverage   82.95%   82.98%   +0.02%     
==========================================
  Files         245      245              
  Lines       25004    25011       +7     
==========================================
+ Hits        20743    20756      +13     
+ Misses       4261     4255       -6     

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[Hywan]

I like how you insert jokes with my name in the tests so that you're checking whether I've reviewed them or not, that's smart 😏.