Provide a top level API for folds

As the fold functions have been added to get_runner in an ad hoc way,
naturally, given the ongoing development of levelEd to support Riak,
it was difficult for a new user (in this case Quviq) to see what folds
are supported, and with what arguments, and expectations.

This PR is for discussion. It is one of many ways to group, spec, and
document the fold functions.

A test is also added for coverage of range queries.

test/end_to_end/recovery_SUITE.erl

@@ -113,11 +113,12 @@ aae_missingjournal(_Config) ->
         fun(B, K, _V, Acc) -> [{B, K}|Acc] end,
     
     {async, AllHeadF1} =
-        leveled_bookie:book_returnfolder(Bookie1,
-                                            {foldheads_allkeys,
-                                                ?RIAK_TAG,
-                                                FoldHeadsFun,
-                                                true, true, false}),
+        leveled_bookie:book_headfold(Bookie1,
+                                     ?RIAK_TAG,
+                                     {FoldHeadsFun, []},
+                                     true,
+                                     true,
+                                     false),
     HeadL1 = length(AllHeadF1()),
     io:format("Fold head returned ~w objects~n", [HeadL1]),