Compaction, retain and recovery

Change the penciller check so that it returns current/replaced/missing not just true/false.

Reduce unnecessary penciller checks for non-standard keys that will always be retained - and remove redunandt code.

Expand tests of retain and recover to make sure that compaction on delete is well covered.

Also move the SQN number laong during initial loads - to stop aggressive loop to find starting SQN every file.

src/leveled_runner.erl

@@ -373,7 +373,7 @@ foldobjects_allkeys(SnapFun, Tag, FoldObjectsFun, sqn_order) ->
                                                                     SQN),
                     % Need to check that we have not folded past the point
                     % at which the snapshot was taken
-                    (JournalSQN >= SQN) and CheckSQN
+                    (JournalSQN >= SQN) and (CheckSQN == current)
                         
                 end,