Convert downloadChapter to suspend function (#9127)

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.

app/src/main/java/eu/kanade/tachiyomi/util/lang/RetryWithDelay.kt

@@ -1,25 +0,0 @@
-package eu.kanade.tachiyomi.util.lang
-
-import rx.Observable
-import rx.Scheduler
-import rx.functions.Func1
-import rx.schedulers.Schedulers
-import java.util.concurrent.TimeUnit.MILLISECONDS
-
-class RetryWithDelay(
-    private val maxRetries: Int = 1,
-    private val retryStrategy: (Int) -> Int = { 1000 },
-    private val scheduler: Scheduler = Schedulers.computation(),
-) : Func1<Observable<out Throwable>, Observable<*>> {
-
-    private var retryCount = 0
-
-    override fun call(attempts: Observable<out Throwable>) = attempts.flatMap { error ->
-        val count = ++retryCount
-        if (count <= maxRetries) {
-            Observable.timer(retryStrategy(count).toLong(), MILLISECONDS, scheduler)
-        } else {
-            Observable.error(error as Throwable)
-        }
-    }
-}