make adding the db.statement tag configurable in jdbc calls

[ivantopo]

This PR makes it possible to decide whether the db.statement tag is going to be added to spans created by the JDBC instrumentation. There are three possible settings:

• always: (default) will always add the db.statement tag to spans
• prepared: will only add the the db.statement tag when the traced execution was made with a PreparedStatement, which should ensure no placeholder values are leaked to the tracing backend
• never: completely disables adding the db.statement tag to spans