Add an iterative process to try the three different API variations for
folding over keys in various backends.  IIRC there's a different
bug/change-request to change these APIs to be more harmonious, so we'll
tackle that bridge when we take it by the horns.

Also added more extensive backend unit tests: for bitcask, cache, dets,
ets, fs, and gb_trees backends.  And added new-style folding API to the
ETS backend, as an example (since it doesn't do anything differently than
the fold implementation).

…d.erl change

Add unit test for the multi backend, which was missing.  Verify
via redbug that each of the actual backends is having the proper
fold function called: ets -> fold_bucket_keys, gb_trees -> fold.

19> redbug:start({riak_kv_ets_backend, fold_bucket_keys}).
ok
20> riak_kv_vnode:list_buckets_multi_backend_test().
ok

18:41:32 <{erlang,apply,2}> {riak_kv_ets_backend,fold_bucket_keys,
                                [<0.206.0>,'_',#Fun<riak_kv_vnode.2.64444309>,
                                 []]}
quitting: timeout
21> redbug:start({riak_kv_gb_trees_backend, fold_bucket_keys}).
ok
22> riak_kv_vnode:list_buckets_multi_backend_test().
ok
23> riak_kv_vnode:list_buckets_multi_backend_test().
ok
24> redbug:stop().
{stop,[]}
quitting: local_done
25> redbug:start({riak_kv_gb_trees_backend, fold}).
ok
26> riak_kv_vnode:list_buckets_multi_backend_test().
ok

18:42:02 <{erlang,apply,2}> {riak_kv_gb_trees_backend,fold,
                                [{state,<0.237.0>},
                                 #Fun<riak_kv_vnode.2.64444309>,
                                 [<<"f">>]]}

Fix Jon's less-than-optimal bug as pointed out in pull req #37.
Also, since the multi backend is actually bucket aware, we save
work by only querying the one backend that we know is responsible
for the bucket.

Verifying, at console:

{ok, C} = riak:local_client().
C:set_bucket(<<"i">>,[{backend, inno}]).
C:set_bucket(<<"b">>,[{backend, bitcask}]).
[C:put(riak_object:new(<<"i">>, <<X:32>>, <<X:32>>)) || X <- lists:seq(1,5)].
[C:put(riak_object:new(<<"b">>, <<X:32>>, <<X:32>>)) || X <- lists:seq(6,10)].
[C:put(riak_object:new(<<"x">>, <<X:32>>, <<X:32>>)) || X <- lists:seq(11,15)].
[{Bucket, C:list_keys(Bucket)} || Bucket <- [<<"i">>, <<"b">>, <<"x">>, <<"none">>]].

Config additions:

--- rel/riak/etc/app.config	2011-02-17 16:50:36.000000000 -0600
+++ /tmp/app.config	2011-02-22 15:01:55.000000000 -0600
@@ -1,6 +1,11 @@
 %% -*- tab-width: 4;erlang-indent-level: 4;indent-tabs-mode: nil -*-
 %% ex: ts=4 sw=4 et
 [
+{innostore, [
+    {data_home_dir, "/tmp/innodb"}, %% Where data files go
+    {log_group_home_dir, "/tmp/innodb"}, %% Where log files go
+    {buffer_pool_size, 268435456} %% 256MB in-memory buffer in bytes
+]},
  %% Riak Core config
  {riak_core, [
               %% Default location of ringstate
@@ -30,7 +35,15 @@
  {riak_kv, [
             %% Storage_backend specifies the Erlang module defining the
             %storage
             %% mechanism that will be used on this node.
-            {storage_backend, riak_kv_bitcask_backend},
+            %%{storage_backend, riak_kv_bitcask_backend},
+{storage_backend, riak_kv_multi_backend},
+{riak_kv_dets_backend_root, "data/dets"},
+{riak_kv_fs_backend_root, "data/fs"},
+{multi_backend_default, default},
+{multi_backend, [{default, riak_kv_dets_backend,
+    [{riak_kv_dets_backend_root, "data/dets"}]},
+     {bitcask, riak_kv_bitcask_backend, []},
+     {inno,    riak_kv_innostore_backend, []}]},

             %% pb_ip is the IP address that the Riak Protocol Buffers
             %interface
             %% will bind to.  If this is undefined, the interface will not
             %run.

([email protected])1> {ok, C} = riak:local_client().
{ok,{riak_client,'[email protected]',<<1,158,32,8>>}}
([email protected])2> C:set_bucket(<<"i">>,[{backend, inno}]).
ok
([email protected])3> C:set_bucket(<<"b">>,[{backend, bitcask}]).
ok
([email protected])4> [C:put(riak_object:new(<<"i">>, <<X:32>>, <<X:32>>)) ||
X <- lists:seq(1,5)].
[ok,ok,ok,ok,ok]
([email protected])5> [C:put(riak_object:new(<<"b">>, <<X:32>>, <<X:32>>)) ||
X <- lists:seq(6,10)].
[ok,ok,ok,ok,ok]
([email protected])6> [C:put(riak_object:new(<<"x">>, <<X:32>>, <<X:32>>)) ||
X <- lists:seq(11,15)].
[ok,ok,ok,ok,ok]
([email protected])7> [{Bucket, C:list_keys(Bucket)} || Bucket <- [<<"i">>,
<<"b">>, <<"x">>, <<"no">>]].
[{<<"i">>,
  {ok,[<<0,0,0,2>>,
       <<0,0,0,5>>,
       <<0,0,0,3>>,
       <<0,0,0,4>>,
       <<0,0,0,1>>]}},
 {<<"b">>,
  {ok,[<<0,0,0,8>>,
       <<0,0,0,7>>,
       <<0,0,0,9>>,
       <<0,0,0,10>>,
       <<0,0,0,6>>]}},
 {<<"x">>,
  {ok,[<<0,0,0,13>>,
       <<0,0,0,12>>,
       <<0,0,0,11>>,
       <<0,0,0,14>>,
       <<0,0,0,15>>]}},
 {<<"no">>,{ok,[]}}]