feat(event cache): don't clear a timeline if storage's enabled

[bnjbvr]

This is one of the last steps for integrating the persistent storage: if a timeline is gappy, then we don't need to reset it, either internally or for observers. That's the whole point of persisting the event cache, after all: we should be able to paginate again from any holes we encounter later.

Part of #3280

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 85.16%. Comparing base (a4434d7) to head (52e6544).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/matrix-sdk/src/event_cache/room/mod.rs	40.00%	3 Missing ⚠️
crates/matrix-sdk/src/event_cache/mod.rs	80.00%	1 Missing ⚠️
crates/matrix-sdk/src/test_utils/mocks.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4374   +/-   ##
=======================================
  Coverage   85.15%   85.16%           
=======================================
  Files         280      280           
  Lines       30826    30831    +5     
=======================================
+ Hits        26251    26256    +5     
  Misses       4575     4575           

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[stefanceriu]

Nice, makes sense to me and all those test tweaks are the cherry on top 👌