Mark Tasks #17 and #18 as complete (apache#44)

Both Subset and SplitByStripe were implemented as part of Task #4.
These methods have been present in the codebase since PR #18.

Author: cbb330

task_list.json

@@ -422,7 +422,7 @@
       "Unit test: Metadata/manifest sharing",
       "Unit test: Fresh cache"
     ],
-    "status": "pending",
+    "status": "complete",
     "depends_on": [13],
     "priority": "P1"
   },
@@ -444,7 +444,7 @@
       "Unit test: Split into per-stripe fragments",
       "Unit test: Metadata sharing"
     ],
-    "status": "pending",
+    "status": "complete",
     "depends_on": [13],
     "priority": "P1"
   },