Deprecated getSession() in DBField and DBObject

Folks should be using getDBSession() instead, or getGSession()
if they want the GanymedeSession related to an object
or field.
Changed DBField.getVectVal() from public to protected

You should use getValuesLocal() instead if you're outside the DBField
hierarchy.
Comments and finalization of some methods in DBField
Added a getIdentityInvid() pass-through in GanymedeSession
Added an internalSingletonQuery() method to GanymedeSession

Just to get rid of the need to do Vector processing when
we're expecting an internalQuery to only return zero or
one results.
Got rid of a non-useful check in GanymedeServer.xmlLogin()
Cleaned up the user session constructor a bit in GanymedeSession
Simplified and reworked login processing
Removed no the no-longer-needed getVal() method from DBField