Evaluate improved sync-load-during-async-load strategy

[zbraniecki]

As per bug https://bugzilla.mozilla.org/show_bug.cgi?id=1723191 we have scenarios where a sync load is triggered in the middle of an async load and we should consider options for handling such scenario better than with https://github.com/mozilla/l10nregistry-rs/blob/master/src/source/mod.rs#L239-L255

With that approach, we just load the one-off synchronously and return it, while we also complete async and we only will cache the async.

Maybe we could hook into the async load, discard it, supply data from the sync load and cache that?

[zbraniecki]

I started looking into this, and the trick here seems to be - how do we cancel an ongoing future in https://github.com/mozilla/l10nregistry-rs/blob/master/src/source/fetcher.rs#L28

We'd like to be able to "drop" a future, and replace it with ResourceStatus resolved in https://github.com/mozilla/l10nregistry-rs/blob/master/src/source/mod.rs#L254 and then "inject" our synchronously loaded data into https://github.com/mozilla/l10nregistry-rs/blob/master/src/source/mod.rs#L24 and resolve it.

Two questions:

1. Would Drop be enough here or do we need to somehow specially cancel a future?
2. Is there a way to "force-resolve" a pending Shared Future from outside?

@emilio - do you have any pointers for me here?