Fix stats since we stops the use of type

[Joxit]

For the supports of ES6, we need to remove the type mapping. We removed the use of types in pelias/model#95 and now, the default value is doc.
This caused the suppression of the statistics when we do import. Instead of the layer, we only have doc=x instead of address=x for OA.

2019-09-18T21:27:28.360Z - info: [dbclient-openaddresses]  paused=false, transient=1, current_length=190, indexed=479421000, batch_ok=958842, batch_retries=0, failed_records=0, doc=479421000, persec=3000

This PR brings statistics back. It will use the layer value instead of the type. I added a fallback on _type just in case.

2019-09-27T09:50:02.852Z - info: [dbclient-openaddresses]  paused=false, transient=0, current_length=441, indexed=21500, batch_ok=43, batch_retries=0, failed_records=0, address=21500, persec=2150

connected to pelias/model#95
connected to pelias/pelias#719

[orangejulius]

Oh yeah, good catch! Thanks!