Add gokv.Store implementation for OrientDB

[philippgille]

OrientDB claims to be the fastest graph database (explicitly mentioning to be faster than Neo4j). It's also a multi-model DB, with the website saying that it supports key-value pairs as well. Maybe just with a document that only has a single value, similar to MongoDB and ArangoDB - I didn't go through the documentation to find this out yet.

• https://github.com/orientechnologies/orientdb
• https://orientdb.org/
• https://github.com/istreamdata/orientgo
• https://godoc.org/gopkg.in/istreamdata/orientgo.v2