[db] Major changes to the way sessions are used in db managers

+ Session is now one per process which can be accessed at Core.DB.session
+ A new Session is created whenever a new owtf process is created since
same connection to db cannot be used by two processes
[db] Implemented sessions and added control in interface

+ Sessions are not serious stuff, ie sessions are only used to filter targets returned by API.
+ Since control flow starts from selecting target(s) in interface this is more than enough
+ Targets which are not in current session will not be visible in target manager but can be accessible
by direct reference