Use new methods in nosql to enable querying for Certificates.

[TheSecMaven]

Description

This resolves #688 and enables users to perform a normal upgrade of smallstep, and the new schema will be migrated to. At which point users can begin to query for certificates the application has signed on a go forward basis. This is reliant on smallstep/nosql#15, and then updating the version of the dependency used in this project.

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[dopey]

smallstep/nosql#15

This relies on changes to the nosql package (PR linked above). We will not merge those changes (reasons given in the PR), therefore this PR would need modification as well.

To recap, we are interested in a SQL implementation of the following interface: https://github.com/smallstep/certificates/blob/master/db/db.go#L45. The existing implementation in open source uses nosql, and is subsequently difficult to query against.