storage: common API that selects appropriate provider given a DSN

[llorllale]

It would be nice to have an API that looks like this:

// mysql provider automatically selected 
provider, err := storage.Open("mysql://root:my-secret-pw@tcp(127.0.0.1:3306)/")
if err != nil {
    // handle error
}

This would reduce refactors users would have to do when selecting different storage providers.

Current obstacles:

• import cycle b/w storage package and implementation-specific packages
• need to pass in implementation-specific options. Perhaps encode these options as url parameters?

[llorllale]

Take a look at what edge-adapter did:

https://github.com/trustbloc/edge-adapter/blob/c8d09fbaf198a8149535ca1ebe6cd766f4864e64/cmd/adapter-rest/startcmd/start.go#L162-L170

https://github.com/trustbloc/edge-adapter/blob/c8d09fbaf198a8149535ca1ebe6cd766f4864e64/cmd/adapter-rest/startcmd/start.go#L625-L666