Do not use the ddoc cache to load _changes filter design documentsfix-changes-filter

Since we started re-using the main changes feed transaction to open the docs
https://github.com/apache/couchdb/commit/1bceb552594af404961e4ab8e6f88cffa1548f69,
we also started to pass the transactional db handle to the ddoc cache. However
ddoc cache uses a `spawn_monitor` to open docs, and since our transaction
objects are tied to the owner process the open was failing with a `badarg`
error from erlfdb.

This could be seen by running the replication elixir tests:

```
make elixir tests=test/elixir/test/replication_test.exs:214
```

1 files changed, 1 insertions, 1 deletions

diff --git a/src/chttpd/src/chttpd_changes.erl b/src/chttpd/src/chttpd_changes.erl
index 81fa90fab..31c39bfb7 100644
--- a/src/chttpd/src/chttpd_changes.erl
+++ b/src/chttpd/src/chttpd_changes.erl
@@ -393,7 +393,7 @@ check_fields(_Fields) ->
 
 
 open_ddoc(Db, DDocId) ->
-    case ddoc_cache:open_doc(Db, DDocId) of
+    case fabric2_db:open_doc(Db, DDocId, [ejson_body, ?ADMIN_CTX]) of
         {ok, _} = Resp -> Resp;
         Else -> throw(Else)
     end.