Move Faunus Graph instantiation into RR.initialize

I kind of implemented this in
5dd656e050cf1f31580a4425e51b3da272d9bfcb, but I cheated by opening the
graph in InputFormat.createRecordReader rather than
RecordReader.initialize.  I'm not sure whether the contract of
createRecordReader makes that safe.  This commit moves graph
instantiation into RecordReader.initialize in an attempt to remove the
uncertainty.

Related to #817