Less memory use for TG import when NamespaceMigrationStep is involved

MigrationStateImpl.getProperty for root resource(s) keys now first
exports CURRENT_TG back to disk if the TG has already been loaded and
then imports it back using StreamingTransferableGraphImportProcess. This
avoids the need to keep the whole TransferableGraph1 in memory while
doing the import and also allows for the use of the leaner Streaming
import process.

refs #7153

Change-Id: I2ef6ae055cd8a497263c3aa3e95c86ff29a96f3e