fix(logger): change default value of log.max_depth to 100

The value 20 is too small to inspect some error messages, especially
when it contains stack traces.

etc/emqx.conf

@@ -443,8 +443,8 @@ log.file = emqx.log
 ## and Erlang process message queue inspection.
 ##
 ## Value: Integer or 'unlimited' (without quotes)
-## Default: 20
-#log.max_depth = 20
+## Default: 100
+#log.max_depth = 100
 
 ## Log formatter
 ## Value: text | json

priv/emqx.schema

@@ -496,7 +496,7 @@ end}.
 %% @doc Maximum depth in Erlang term log formatting
 %% and message queue inspection.
 {mapping, "log.max_depth", "kernel.error_logger_format_depth", [
-  {default, 20},
+  {default, 100},
   {datatype, [{enum, [unlimited]}, integer]}
 ]}.