Merge 'sorter: fix sorter panic on SortedChunkIOState::WaitingForRead' from Jussi Saurio

Closes #2165
The following sequence of events is possible:
- init_chunk_heap() called
- flush() gets called, and all chunks start writing to disk
- chunk A status is WriteComplete, so chunk.read() gets called on chunk
A
- chunk A sets its status to WaitingForRead
- some other chunk B is still in WaitingForWrite status after flush()
- for this reason, init_chunk_heap() returns IOResult::IO
- init_chunk_heap() is called again
- we panic because chunk A is in WaitingForRead status
So - we just allow WaitingForRead status in init_chunk_heap() instead.
This panic was caught thanks to Pedro's IO latency enhancement to the
sim!

Reviewed-by: Iaroslav Zeigerman (@izeigerman)

Closes #2166

core/vdbe/sorter.rs

@@ -149,7 +149,7 @@ impl Sorter {
                     // Write complete, we can now read from the chunk.
                     chunk.read()?;
                 }
-                SortedChunkIOState::WaitingForWrite => {
+                SortedChunkIOState::WaitingForWrite | SortedChunkIOState::WaitingForRead => {
                     all_read_complete = false;
                 }
                 SortedChunkIOState::ReadEOF | SortedChunkIOState::ReadComplete => {}