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_penciller.erl

@@ -315,21 +315,22 @@ pcl_fetchlevelzero(Pid, Slot) ->
 %% The Key needs to be hashable (i.e. have a tag which indicates that the key
 %% can be looked up) - index entries are not hashable for example.
 %%
-%% If the hash is already knonw, call pcl_fetch/3 as magic_hash is a
+%% If the hash is already knonw, call pcl_fetch/3 as segment_hash is a
 %% relatively expensive hash function
 pcl_fetch(Pid, Key) ->
-    Hash = leveled_codec:magic_hash(Key),
+    Hash = leveled_codec:segment_hash(Key),
     if
         Hash /= no_lookup ->
             gen_server:call(Pid, {fetch, Key, Hash}, infinity)
     end.
 
--spec pcl_fetch(pid(), tuple(), integer()) -> {tuple(), tuple()}|not_present.
+-spec pcl_fetch(pid(), tuple(), {integer(), integer()}) -> 
+                                            {tuple(), tuple()}|not_present.
 %% @doc
 %% Fetch a key, return the first (highest SQN) occurrence of that Key along
 %% with  the value.
 %%
-%% Hash should be result of leveled_codec:magic_hash(Key)
+%% Hash should be result of leveled_codec:segment_hash(Key)
 pcl_fetch(Pid, Key, Hash) ->
     gen_server:call(Pid, {fetch, Key, Hash}, infinity).
 
@@ -367,7 +368,7 @@ pcl_fetchnextkey(Pid, StartKey, EndKey, AccFun, InitAcc) ->
 %% If the key is not present, it will be assumed that a higher sequence number
 %% tombstone once existed, and false will be returned.
 pcl_checksequencenumber(Pid, Key, SQN) ->
-    Hash = leveled_codec:magic_hash(Key),
+    Hash = leveled_codec:segment_hash(Key),
     if
         Hash /= no_lookup ->
             gen_server:call(Pid, {check_sqn, Key, Hash, SQN}, infinity)
@@ -1317,7 +1318,7 @@ generate_randomkeys(Count, SQN, Acc) ->
     RandKey = {K,
                 {SQN,
                 {active, infinity},
-                leveled_codec:magic_hash(K),
+                leveled_codec:segment_hash(K),
                 null}},
     generate_randomkeys(Count - 1, SQN + 1, [RandKey|Acc]).
     
@@ -1347,7 +1348,7 @@ maybe_pause_push(PCL, KL) ->
     T1 = lists:foldl(fun({K, V}, {AccSL, AccIdx, MinSQN, MaxSQN}) ->
                             UpdSL = [{K, V}|AccSL],
                             SQN = leveled_codec:strip_to_seqonly({K, V}),
-                            H = leveled_codec:magic_hash(K),
+                            H = leveled_codec:segment_hash(K),
                             UpdIdx = leveled_pmem:prepare_for_index(AccIdx, H),
                             {UpdSL, UpdIdx, min(SQN, MinSQN), max(SQN, MaxSQN)}
                             end,
@@ -1366,7 +1367,7 @@ maybe_pause_push(PCL, KL) ->
 
 %% old test data doesn't have the magic hash
 add_missing_hash({K, {SQN, ST, MD}}) ->
-    {K, {SQN, ST, leveled_codec:magic_hash(K), MD}}.
+    {K, {SQN, ST, leveled_codec:segment_hash(K), MD}}.
 
 
 clean_dir_test() ->