This fixes a bug where that clients using the legacy approach of a
"very large anchor" value with the intent to only get the most recent
messages would only get found_newest=True if they used the specific
value LARGER_THAN_MAX_MESSAGE_ID. Now any value at least that large
will work.
In upcoming commits, we plan to replace this with passing the string
"last", but it seems worth removing the buggy "special value" behavior
while we're touching this code.
e2810d7549