Refactor MediaCache

[aaravlu]

pub struct MediaCache {
    /// The actual cached data.
    cache: BTreeMap<OwnedMxcUri, MediaCacheEntryRef>,
    /// The default format to use when fetching media.
    default_format: MediaFormatConst,
    /// A channel to send updates to a particular timeline when a media request has completed.
    timeline_update_sender: Option<crossbeam_channel::Sender<TimelineUpdate>>,
}

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For images in the matrix server, firstly, they are divided into two segments: the original and the thumbnail.

Then, separately, there are several versions with different resolutions under the two segments: 100x100, 200x200 ......, that is, there are many combinations.

Currently, Robrix is using a combination in the timeline: thumbnail & 400x400

But, there is a rare condition: an image is unclear enough, it would not possess a thumbnail, Ok ,we show the combination in the timeline: original & 400x400.

When user click it (the rare condition mentioned above), try_get_media_or_fetch wont fetch the combination: original & File, instead, it would find the image data (original & 400x400) via same key OwnedMxcUri in local disk, and show it, this is not what we want.

---

This refactoring is aimed to solve this rare problem while improving its robustness.
I shall advance #341 after this issue is fixed.