Implement database span semantic convention updates

[justinfoote]

The semantic conventions for database spans was recently updated. See:
open-telemetry/opentelemetry-specification#575

The instrumentation needs to be updated with these new attributes.

Primary among them is db.system, which applies to all database spans.
Also some connection-level attributes and the call-level attributes:

• db.name
• db.statement
• db.operation

[heyams]

Please assign this to me. Thank you!