* Remove one sql query from user_admin which was not needed.

* Remove one sql query from user_admin which was not needed.
* Instead of a query like "LIKE 'YYYY-MM-DD%'" use "AcctStopTime >= 'YYYY-MM-DD 00:00:00 AND AcctStopTime
  <= 'YYYY-MM-DD 23:59:59'" which will allow us to use sql indexes better.
* Add a few comments in bin/clean_radacct
* Add a new script bin/truncate_radacct which will delete all sessions from the radacct table which
  are older than a configurable number of days.
* Add an entry in tuning_guide about creating a multi column index for (UserName,AcctStopTime). This
  will really help sql_checksimul and the corresponding queries.