Initial head_only features

Initial commit to add head_only mode to leveled.  This allows leveled to receive batches of object changes, but where those objects exist only in the Penciller's Ledger (once they have been persisted within the Ledger).

The aim is to reduce significantly the cost of compaction.  Also, the objects ar enot directly accessible (they can only be accessed through folds).  Again this makes life easier during merging in the LSM trees (as no bloom filters have to be created).

src/leveled_iclerk.erl

@@ -82,6 +82,7 @@
         clerk_new/1,
         clerk_compact/7,
         clerk_hashtablecalc/3,
+        clerk_trim/3,
         clerk_stop/1,
         code_change/3]).      
 
@@ -144,6 +145,12 @@ clerk_compact(Pid, Checker, InitiateFun, CloseFun, FilterFun, Inker, TimeO) ->
                     Inker,
                     TimeO}).
 
+-spec clerk_trim(pid(), pid(), integer()) -> ok.
+%% @doc
+%% Trim the Inker back to the persisted SQN
+clerk_trim(Pid, Inker, PersistedSQN) ->
+    gen_server:cast(Pid, {trim, Inker, PersistedSQN}).
+
 -spec clerk_hashtablecalc(ets:tid(), integer(), pid()) -> ok.
 %% @doc
 %% Spawn a dedicated clerk for the process of calculating the binary view
@@ -235,6 +242,12 @@ handle_cast({compact, Checker, InitiateFun, CloseFun, FilterFun, Inker, _TO},
             ok = CloseFun(FilterServer),
             {noreply, State}
     end;
+handle_cast({trim, Inker, PersistedSQN}, State) ->
+    [_Active|Manifest] = leveled_inker:ink_getmanifest(Inker),
+    FilesToDelete = 
+        leveled_imanifest:find_persistedentries(PersistedSQN, Manifest),
+    ok = update_inker(Inker, [], FilesToDelete),
+    {noreply, State};
 handle_cast({hashtable_calc, HashTree, StartPos, CDBpid}, State) ->
     {IndexList, HashTreeBin} = leveled_cdb:hashtable_calc(HashTree, StartPos),
     ok = leveled_cdb:cdb_returnhashtable(CDBpid, IndexList, HashTreeBin),
@@ -527,7 +540,7 @@ update_inker(Inker, ManifestSlice, FilesToDelete) ->
                                                         Inker)
                         end,
                     FilesToDelete),
-                    ok.
+    ok.
 
 compact_files(BestRun, CDBopts, FilterFun, FilterServer, 
                                             MaxSQN, RStrategy, PressMethod) ->