MDEV-20854: ANALYZE for statements: not clear where the time is spent

Count the "gap" time between table accesses and display it as
r_other_time_ms in the "table" element.

* The advantage of this approach is that it doesn't add any new
  my_timer_cycles() calls.
* The disadvantage is that the definition of what is done during
  "other time" is not that clear: it includes checking the WHERE
  (for this table), constructing index lookup tuple (for the next table)
  writing to GROUP BY temporary table (as we dont account for that time
  separately [yet], etc)

sql/sql_class.cc

@@ -1336,6 +1336,7 @@ void THD::init()
 #endif //EMBEDDED_LIBRARY
 
   apc_target.init(&LOCK_thd_kill);
+  gap_tracker_data.init();
   DBUG_VOID_RETURN;
 }