Improve Virtual Topic performance

Description - Performance of VTs are very bad compared to Queues and TopicsThis was discussed with Gary Tully and looks like virtual topics are slower because of the design,
For Virtual Topics with 1 pub to 10 subs there would be 10 writes for each queue and in case of NFS these messages have to be written and synced each time.Suggested changes -
A. Write the message only once and the queues subscribed gain a copy from there. This is like levelDB.
B. If concurrent sends on composites are faster but composites are not ok for sutter, we can try to implement that logic for vt's.