Simplify SessionContext

the current way to implement a SessionContext is IMO over-engineered.
a simple thing like a container used to pass around stuff associated
with a session shouldn't cause headache such as the current implementation
does using a derived interface and the original not even being used.

while we are at this:
jukka once suggested to use an adapter class in order not to expose
the sessionImpl altogether on the JCR level. that might be a next step to go.