echonest: don't needlessly write during import

On the import hook, the config['import']['write'] check (and writing at all)
is not necessary, since this will be taken care of later in the program.
echonest: use pipeline stage

In addition to the performance benefit of running in our own thread, this also
obviates the need for the _songs dict to pass stuff between stages.
echonest: simplify method-aggregator loop