Include cached hash code in TransferableGraph1 extensions map

The cached hashcode is calculated before inserting the cached hash value
in the TransferableGraph1.extensions TreeMap. Reading and inspection
must be done with this in mind.

This cached value is used to optimize platform startup by removing the
need to calculate the TransferableGraph1 structure hashcode at runtime
which can be quite expensive because the standard databoard Bindings are
slow compared to optimal methods. Of course an optimized binding could
be used to speed the calculation up dramatically but it still can't beat
caching the final (immutable) hashCode value.

refs #7806

Change-Id: I9d4fc6549cba567f95b19f80bd15c1ccfbbedf07