WL#2731: Multi-source: SHOW SLAVE STATUS

SUMMARY

1. Add command: SHOW SLAVE STATUS FOR ALL MASTERS
   
   This was planned as part of WL#1697, but is not included 
   in the current patch.

2. For SHOW SLAVE STATUS, retried_trans should be reported.
   (In the current patch, the retried_trans variable shows
   the sum of all retired trans for all named masters, in
   SHOW SLAVE STATUS it should be possible to get this 
   value for a single one.)