supported multiple result sets in *sql.Rows result from db.QueryContext

[asmyasnikov]

What this PR does / why we need it:
Since go1.8 *sql.Rows supports multiple result sets. But grafana-plugin-sdk-go doesn't take this into.
This PR improved grafana-plugin-sdk-go for supporting multiple result sets in db.QueryContext result

Which issue(s) this PR fixes:
Some databases (like YDB) have stream protocol inside. Because designed for large data sets. This fact determines stream API for getting result from server. Multiple result sets is normal for iterate between result batches

Fixes #

Special notes for your reviewer:

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[Raubzeug]

Hi! Could you please provide with further steps to merge this PR and deploy a new version? YDB team is waiting for this improvement to fix a bug in our Grafana plugin.
Thanks in advance!