Add gokv.Store implementation for CockroachDB

[philippgille]

CockroachDB is one of the most popular "NewSQL" databases. It's inspired by Google Spanner and supports strong consistency while also being horizontally scalable and robust (data is replicated so a node can crash without any data being lost).

• https://www.cockroachlabs.com/
• https://www.cockroachlabs.com/docs/stable/start-a-local-cluster-in-docker.html#os-linux
• https://www.cockroachlabs.com/docs/stable/learn-cockroachdb-sql.html
• https://www.cockroachlabs.com/docs/stable/build-a-go-app-with-cockroachdb.html

Regarding usage as key-value store:

• https://www.cockroachlabs.com/docs/stable/frequently-asked-questions.html#can-i-use-cockroachdb-as-a-key-value-store
• https://forum.cockroachlabs.com/t/can-i-use-cockroachdb-as-a-kv-store/56
• https://github.com/cockroachdb/docs/blob/b68c9ad8097d1efec4d2b6d849f6788a0e857215/v2.1/column-families.md