Possible fix to getting thrown to the bottom while reading unreads.

Shoutout to @fumiakiy for the excellent research here!

Sometimes we get thrown to the bottom of the list (or other list
locations) when reading content in the middle of the list. Most often,
this happens when you have a lot of unread messages and you open the
conversation.

FixedSizePagingController#onDataNeededAroundIndex() can be called very
fast in rapid succession, and we use the DataStatus class for
bookkeeping to know which requests are in-flight. We then make those
requests in LIFO order in order to make sure that the data visible on
screen now gets the highest priority.

...But in practice, that LIFO ordering can make things a little screwy.
Imagine we called onDataNeedAroundIndex() 50 times in rapid succession
(1, 2..., 50). Each time it's called, we generate a range and mark that
range as being fetched in DataStatus. That could mean that the latest
request for index 50 might only have, like, 1 item in it, because a
previously-enqueued fetch already got assigned most of it's data.

BUT we execute the nearly-empty request for index 50 first because of the
LIFO ordering. We give that data to RecyclerView first, and it doesn't like
that at all, and it jumps to weird places because we gave it mostly
null values, which are rendered as placeholder values (which are smaller
than real cells). So then, when we give it the real data right after,
its position is all off.

I switched to a serial executor. That prevents us from giving back weird
lists. The consequence is that if you scroll super fast, you run the
risk of the executor getting 'backed up' fetching data that's offscreen.
However, in practice, I couldn't trigger this. We'll see how it goes. I
think the true solution is a smarter way of fetching and ordering
requests, but that gets to be really tricky from a threading
perspective, and I'd rather keep things simple.

paging/lib/src/main/java/org/signal/paging/FixedSizePagingController.java

@@ -22,7 +22,7 @@ class FixedSizePagingController<E> implements PagingController {
 
   private static final String TAG = FixedSizePagingController.class.getSimpleName();
 
-  private static final Executor FETCH_EXECUTOR = SignalExecutors.newFixedLifoThreadExecutor("signal-FixedSizePagingController", 1, 1);
+  private static final Executor FETCH_EXECUTOR = SignalExecutors.newCachedSingleThreadExecutor("signal-FixedSizePagingController");
   private static final boolean  DEBUG          = false;
 
   private final PagedDataSource<E>       dataSource;
@@ -49,7 +49,7 @@ class FixedSizePagingController<E> implements PagingController {
   /**
    * We assume this method is always called on the same thread, so we can read our
    * {@code loadState} and construct the parameters of a fetch request. That fetch request can
-   * then be performed on separate single-thread LIFO executor.
+   * then be performed on separate single-thread executor.
    */
   @Override
   public void onDataNeededAroundIndex(int aroundIndex) {