1:1 translation from the RxJava implementation, should match the
previous behavior.
Dropped the return value from functions of the form
```
fun foo(t: T, ...): Observable<T>
```
where the Observable produced the original argument `t`.
The caller already has the result if necessary.
While this conversion is not flow-based overall, some sections use
flows to use the flatMapMerge and retryWhen operators.
Removed RetryWithDelay as it was only used here.
Inlined fetchAllImageUrlsFromPageList instead of converting it to a
suspending equivalent. fetchAllImageUrlsFromPageList is no longer
used in the app, but was not removed as it is part of source-api.
(However, it does not seem to be used exposed in extensions-lib or
used in tachiyomi-extensions.)
runBlocking is used as a temporary stop-gap.
(cherry picked from commit fa61c8fe6fb4966f5d23f643759f0e4229b8f016)
# Conflicts:
# app/src/main/java/eu/kanade/tachiyomi/data/download/Downloader.kt
87107fcc94