Change calling contract for ModelLoaderBase.readTable(tablename)

Now it must be called with tablename in the correct case.
Only caller left (after cleanup from 47036) in readTables()
Modify the (pg specific) query within to return the proper case
That allows to remove the extra query done in PG.readTable to find the
proper case (_stmt_oids_tables). Note that the 2nd field queried in
there before (table oid) was made obsolete by the previous commit also.

Runtime of Reading tables ... for part1..3 (on example i3-4130)
Before: 8.7 .. 9.7 seconds
After:  5.8 seconds