Change hash algorithm for penciller

Switch from magic hash to md5 - to hopefully remove the need for some
of the artificial jumps required to get expected fall positive ratios.

Also split the hash into two 16-bit integers.  We assume that SegmentID
(from the perspective of AAE merkle/tictac trees) will always be at
least 16 bits.  the idea is that hashes should be used in blooms and
indexes such that some advantage can be gained from just knowing the
segmentID - in particular when folding over all the keys in a bucket.

Performance testing has been difficult so far - I think due to “cloud”
mysteries.

src/leveled_codec.erl

@@ -65,6 +65,7 @@
         integer_now/0,
         riak_extract_metadata/2,
         magic_hash/1,
+        segment_hash/1,
         to_lookup/1]).         
 
 -define(V1_VERS, 1).
@@ -79,6 +80,20 @@
                             integer()|null, % Hash of vclock - non-exportable 
                             integer()}. % Size in bytes of real object
 
+
+-spec segment_hash(any()) -> {integer(), integer()}.
+%% @doc
+%% Return two 16 bit integers - the segment ID and a second integer for spare
+%% entropy.  The hashed should be used in blooms or indexes such that some 
+%% speed can be gained if just the segment ID is known - but more can be 
+%% gained should the extended hash (with the second element) is known
+segment_hash(Key) when is_binary(Key) ->
+    <<SegmentID:16/integer, ExtraHash:16/integer, _Rest/binary>> = 
+        crypto:hash(md5, Key),
+    {SegmentID, ExtraHash};
+segment_hash(Key) ->
+    segment_hash(term_to_binary(Key)).
+
 -spec magic_hash(any()) -> integer().
 %% @doc 
 %% Use DJ Bernstein magic hash function. Note, this is more expensive than
@@ -87,10 +102,6 @@
 %% Hash function contains mysterious constants, some explanation here as to
 %% what they are -
 %% http://stackoverflow.com/questions/10696223/reason-for-5381-number-in-djb-hash-function
-magic_hash({?RIAK_TAG, Bucket, Key, _SubKey}) ->
-    magic_hash({Bucket, Key});
-magic_hash({?STD_TAG, Bucket, Key, _SubKey}) ->
-    magic_hash({Bucket, Key});
 magic_hash({binary, BinaryKey}) ->
     H = 5381,
     hash1(H, BinaryKey) band 16#FFFFFFFF;
@@ -516,7 +527,9 @@ parse_date(LMD, UnitMins, LimitMins, Now) ->
 
 -spec generate_ledgerkv(
             tuple(), integer(), any(), integer(), tuple()|infinity) ->
-            {any(), any(), any(), {integer()|no_lookup, integer()}, list()}.
+            {any(), any(), any(), 
+                {{integer(), integer()}|no_lookup, integer()}, 
+                list()}.
 %% @doc
 %% Function to extract from an object the information necessary to populate
 %% the Penciller's ledger.
@@ -537,7 +550,7 @@ generate_ledgerkv(PrimaryKey, SQN, Obj, Size, TS) ->
                     _ ->
                         {active, TS}
                 end,
-    Hash = magic_hash(PrimaryKey),
+    Hash = segment_hash(PrimaryKey),
     {MD, LastMods} = extract_metadata(Obj, Size, Tag),
     ObjHash = get_objhash(Tag, MD),
     Value = {SQN,