page_cache: replace long mutex sleep with SeqWait

When calling into the page cache, it was possible to wait on a blocking
mutex, which can stall the async executor.

Replace that sleep with a SeqWait::wait_for(lsn).await so that the
executor can go on with other work while we wait.

Change walreceiver_works to an AtomicBool to avoid the awkwardness of
taking the lock, then dropping it while we call wait_for and then
acquiring it again to do real work.

Cargo.lock

@@ -1339,6 +1339,7 @@ dependencies = [
  "tokio-stream",
  "tui",
  "walkdir",
+ "zenith_utils",
 ]
 
 [[package]]