SystemSession#createSession should return SessionImpl again

a long with the fix of JCR-2890 (revision 1089436) the behavior of SystemSession#createSession has changed to
return a SystemSession instead of SessionImpl as it used to be.

while i basically consider this move to be correct and the better way of dealing with that session-cloning
mechanism as it prevents the user of this method to convert a SystemSession into a regular session
for extra writing operations (such as e.g. access control editing that is not supported with the
system session to prevent chicken-egg-problems on repo startup).

therefore i would like to revert that change for the 2.4 release in order to prevent regressions.

for the time after 2.4 i would however suggest that we finally take the time to clearly define the
usages, abilities and responsibilities of the system session and also review how and where we
expose them to the individual 'modules' of jackrabbit core.. i started working on this but decided
that this is definitely too risky for 2.4 whereas reverting the change mentioned above should
imo impose very limited risk as all usages of those sessions i am aware of use them as "Session"
or "SesssionImpl", most of them not even having access to the SystemSession class.