bootstrap: check more metadata when loading the snapshot

This patch stores the metadata about the Node.js binary into the SnapshotData and adds fields denoting how the snapshot was generated, on what platform it was generated as well as the V8 cached data version flag. Instead of simply crashing when the metadata doesn't match, Node.js now prints an error message and exit with 1 for the customized snapshot, or ignore the snapshot and start from scratch if it's the default one.