Add switch-to-new-search command

Add the switch-to-new-search command under the search command. This is
used when migrating from legacy riak_search to yokozuna. It does a
multicall across the cluster handing over HTTP and PB control from
riak_search to yokozuna.

This is needed for #20.

## Test Instructions ##


Create a devrel.

Enabled search on all nodes:

```
sed -e 's/search = off/search = on/' -i.bak dev/dev*/etc/riak.conf
```

Enable legacy riak_search on all nodes. First create advanced.config file.

```
[
 {riak_search, [{enabled, true}]}
].
```

Then copy to all nodes.

```
for d in dev/dev*; do cp -v ~/advanced.config $d/etc/; done
```

Start 3 nodes.

```
for d in dev/dev{1,2,3}; do $d/bin/riak start; done
```

Create a 3 node cluster.

```
for d in dev/dev{2,3}; do $d/bin/riak-admin cluster join dev1@127.0.0.1; done
./dev/dev1/bin/riak-admin cluster plan
./dev/dev1/bin/riak-admin cluster commit
```

Wait for transfers to finish.

```
./dev/dev1/bin/riak-admin transfers
```

Attach to console and verify that legacy riak_search controls the HTTP
and PB services.

```
(dev1@127.0.0.1)2> {Res, []} = rpc:multicall(webmachine_router, get_routes, []).
{[[{["search","schema",schema],yz_wm_schema,[]},
   {["search","index"],yz_wm_index,[]},
...
(dev1@127.0.0.1)3> [lists:keyfind(["solr",index,"select"], 1, R) || R <- Res].
[{["solr",index,"select"],riak_solr_searcher_wm,[]},
 {["solr",index,"select"],riak_solr_searcher_wm,[]},
 {["solr",index,"select"],riak_solr_searcher_wm,[]}]
(dev1@127.0.0.1)7> rpc:multicall(riak_api_pb_registrar, lookup, [27]).
{[{ok,riak_search_pb_query},
  {ok,riak_search_pb_query},
  {ok,riak_search_pb_query}],
 []}
(dev1@127.0.0.1)8> rpc:multicall(riak_api_pb_registrar, lookup, [28]).
{[{ok,riak_search_pb_query},
  {ok,riak_search_pb_query},
  {ok,riak_search_pb_query}],
 []}
```

Switch to new search (Yokozuna).

```
./dev/dev1/bin/riak-admin search switch-to-new-search
```

Attach to console and verify that Yokozuna controls HTTP and PB
services.

```
(dev1@127.0.0.1)9> {Res2, []} = rpc:multicall(webmachine_router, get_routes, []).
{[[{["solr",index,"select"],yz_wm_search,[]},
...
(dev1@127.0.0.1)10> [lists:keyfind(["solr",index,"select"], 1, R) || R <- Res2].  
[{["solr",index,"select"],yz_wm_search,[]},
 {["solr",index,"select"],yz_wm_search,[]},
 {["solr",index,"select"],yz_wm_search,[]}]
(dev1@127.0.0.1)11> rpc:multicall(riak_api_pb_registrar, lookup, [27]).           
{[{ok,yz_pb_search},{ok,yz_pb_search},{ok,yz_pb_search}],[]}
(dev1@127.0.0.1)12> rpc:multicall(riak_api_pb_registrar, lookup, [28]).           
{[{ok,yz_pb_search},{ok,yz_pb_search},{ok,yz_pb_search}],[]}
```