Add singleSession attribute to @SendToUser

Added the ability to send a message only to one user session. Given a
user has two tabs open and the client sends a message to the server
from tab 1, it is now possible to send a reply message to only 1 tab
instead of the default mode of targetting all known user sessions.

Issue: SPR-11506
Update @SendToUser and related code

Issue: SPR-11506
Merge pull request #487 from cloudmark/SPR-11506