Allow for segment-acceleration of folds

Initially with basic tests.  If the SlotIndex has been cached, we can now use the slot index as it is based on the Segment hash algortihm.

This looks like it should lead to an order of magnitude improvement in querying for keys/clocks by segment ID.

This also required a slight tweak to the penciller keyfolder.  It now caches the next answer from the SSTiter, rather than restart the iterator.   When the IMMiter has many more entries than the SSTiter (as the sSTiter is being filtered but not the IMMiter) this could lead to lots of repeated folding.

test/end_to_end/riak_SUITE.erl

@@ -100,8 +100,7 @@ perbucket_aae(_Config) ->
         {foldheads_allkeys,
             ?RIAK_TAG,
             {get_segment_folder(DLs, TreeSize),  []},
-            false,
-            true},
+            false, true},
     
     SW_SL0 = os:timestamp(),
     {async, Book2SegFolder} =
@@ -115,7 +114,27 @@ perbucket_aae(_Config) ->
     io:format("Segment lists found ~w ~w~n", [Book2SegList, Book3SegList]),
 
     Delta = lists:subtract(Book2SegList, Book3SegList),
-    true = length(Delta) == 1.
+    true = length(Delta) == 1,
+    
+    SuperHeadSegmentFolder = 
+        {foldheads_allkeys,
+            ?RIAK_TAG,
+            {get_segment_folder(DLs, TreeSize),  []},
+            false, true, DLs},
+    
+    SW_SL1 = os:timestamp(),
+    {async, Book2SegFolder1} =
+        leveled_bookie:book_returnfolder(Bookie2, SuperHeadSegmentFolder),
+    {async, Book3SegFolder1} =
+        leveled_bookie:book_returnfolder(Bookie3, SuperHeadSegmentFolder),
+    Book2SegList1 = Book2SegFolder1(),
+    Book3SegList1 = Book3SegFolder1(),
+    Time_SL1 = timer:now_diff(os:timestamp(), SW_SL1)/1000,
+    io:format("Two segment list folds took ~w milliseconds ~n", [Time_SL1]),
+    io:format("Segment lists found ~w ~w~n", [Book2SegList1, Book3SegList1]),
+
+    Delta1 = lists:subtract(Book2SegList1, Book3SegList1),
+    true = length(Delta1) == 1.
 
 
 get_segment_folder(SegmentList, TreeSize) ->