Khepri: Use queue projection for rabbit_db_queue:get/1

This change uses the Khepri projection for queues for
rabbit_db_queue:get/1 and rabbit_db_queue:get_durable/1.
Switching to the projection here for 'get's improves performance for
any operation that needs to look up a queue. For example, importing
a set of 100k classic queues takes a noticeably smaller amount of time.

| Q getting function  | Time to import 100k classic queues (secs) |
|---                  |---                                        |
| rabbit_khepri:get/1 | 19.20                                     |
| ets:lookup/2        | 15.71                                     |
| (mnesia)            | 14.96                                     |

Author: the-mikedavis

deps/rabbit/src/rabbit_db_queue.erl

@@ -450,9 +450,9 @@ get_in_mnesia(Name) ->
     rabbit_mnesia:dirty_read({?MNESIA_TABLE, Name}).
 
 get_in_khepri(Name) ->
-    case rabbit_khepri:get(khepri_queue_path(Name)) of
-        {ok, Q} -> {ok, Q};
-        _       -> {error, not_found}
+    case ets:lookup(?KHEPRI_PROJECTION, Name) of
+        [Q] -> {ok, Q};
+        []  -> {error, not_found}
     end.
 
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