KTOR-7845 Fix for threading issue in flushAndClose for reader job channels

[bjhham]

Subsystem
Core, I/O

Motivation
KTOR-7845 File is not commited after closing writeChannel() of the file

Solution
The problem here is that flushAndClose() doesn't wait for the ReaderJob to finish so it has no effect on the final destination of the data. By adding a hook to wait for the job to complete, we can daisy chain these jobs with some trust that flushAndClose() will work properly from the perspective of the caller.

For reference, here is the writeChannel utility function:

@OptIn(DelicateCoroutinesApi::class)
public fun File.writeChannel(
    coroutineContext: CoroutineContext = Dispatchers.IO
): ByteWriteChannel = GlobalScope.reader(CoroutineName("file-writer") + coroutineContext, autoFlush = true) {
    @Suppress("BlockingMethodInNonBlockingContext")
    RandomAccessFile(this@writeChannel, "rw").use { file ->
        val copied = channel.copyTo(file.channel)
        file.setLength(copied) // truncate tail that could remain from the previously written data
    }
}.channel

[e5l]

Please change the target branch to 3.1.0