Use async slices for filesystem and block device operations.

Ensure that overlapping I/O operations are correctly visualized.

Change-Id: I486ecef8e157e3c08f6fe26cb761fdadacdd037b
diff --git a/.gitignore b/.gitignore
index bdc160b..0a060bc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 config.pyc
+trace.html