Implement statement import for archive support

[vyzo]

Implements the /import API endpoint, which verifies and merges a stream of existing statements.
Closes #134

Example:

$ mcclient id QmeiY2eHMwK92Zt6X4kUUC3MsjMmVb2VnGZ17DhnhRPCEQ
Peer ID: QmeiY2eHMwK92Zt6X4kUUC3MsjMmVb2VnGZ17DhnhRPCEQ
Publisher ID: 4XTTM4K8sqTb7xYviJJcRDJ5W6TpQxMoJ7GtBstTALgh5wzGm
Info: Metadata for CC images from DPLA, 500px, and pexels; operated by Mediachain Labs.
$ mcclient query -r QmeiY2eHMwK92Zt6X4kUUC3MsjMmVb2VnGZ17DhnhRPCEQ "SELECT * FROM images.500px LIMIT 1000" > /tmp/500px.json
$ curl --data-binary @/tmp/500px.json http://127.0.0.1:9002/import
1000
$ mcclient query "SELECT COUNT(*) FROM images.500px"
1000
$ mcclient query -r QmeiY2eHMwK92Zt6X4kUUC3MsjMmVb2VnGZ17DhnhRPCEQ "SELECT * FROM images.500px LIMIT 5000" > /tmp/more-500px.json
$ curl --data-binary @/tmp/more-500px.json http://127.0.0.1:9002/import
4000
$ mcclient query "SELECT COUNT(*) FROM images.500px"
5000