MXS-2592 Add configuration for session specific in-memory log

When enabled each session will write log messages in the in-memory log.
If session ends in error this log is written to the actual log in disk.

maxctrl/lib/show.js

@@ -59,7 +59,8 @@ const session_fields = [
     {'Idle': 'attributes.idle'},
     {'Connections': 'attributes.connections[].server'},
     {'Connection IDs': 'attributes.connections[].protocol_diagnostics.connection_id'},
-    {'Queries': 'attributes.queries[].statement'}
+    {'Queries': 'attributes.queries[].statement'},
+    {'Log': 'attributes.log'}
 ]
 
 const filter_fields = [