Handle stats db conflicts

Since a timestamp is used as the document id, it's possible for e.g.
multiple nodes to attempt to insert the same document, resulting in
logged errors like the following:

[error] 2022-04-05T21:00:35.302496Z node1@127.0.0.1 emulator -------- Error in process <0.588.159> on node 'node1@127.0.0.1' with exit value:
{{nocatch,conflict},[{fabric,update_doc,3,[{file,"src/fabric.erl"},{line,332}]},{ioq_server,save_to_db,0,[{file,"src/ioq_server.erl"},{line,529}]}]}

This catches cases of `error:conflict` and logs them at info level
instead.

src/ioq_server.erl

@@ -527,8 +527,11 @@ save_to_db() ->
         ]},
         try
             fabric:update_doc(get_stats_dbname(), Doc, [])
-        catch error:database_does_not_exist ->
-            couch_log:debug("Missing IOQ stats db: ~s", [get_stats_dbname()])
+        catch
+            error:database_does_not_exist ->
+                couch_log:debug("Missing IOQ stats db: ~s", [get_stats_dbname()]);
+            error:conflict ->
+                couch_log:info("~p:save_to_db conflict saving ~p", [?MODULE, Doc])
         end
     after Timeout ->
         error_logger:error_report({?MODULE, "ets transfer failed"})