emsmdb: create only 1 emsmdb context per user

Still if the user dies without sendint the proper disconnect the session won't
be released.
nspi: release stalled sessions

Set to 8 hours, nspi sessions appears to only interact to server on demand.
This probably needs tweaking, maybe config key to specify this value?
emsmdb: release stalled sessions

Set to 2 minutes, outlook client ping every few seconds. I'm sure this
will need tweaking, maybe config key to specify this value?

Check another outlook clients and configs. Tested Outlook 2007 directly using MAPI