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_explain.h

@@ -835,6 +835,8 @@ public:
   /* Tracker for reading the table */
   Table_access_tracker tracker;
   Exec_time_tracker op_tracker;
+  Gap_time_tracker extra_time_tracker;
+
   Table_access_tracker jbuf_tracker;
   
   Explain_rowid_filter *rowid_filter;