bridge tables: be more conservative over data wrangling

[peterdutey]

Bridge table functions should be improved to only work on a predictable set of fields.
See for instance here

In a particular instance where encounter_id was included in the drug_prescriptions table, the left join with inpatient_episodes would create fields encounter_id.x and encounter_id.y and break the execution.
This would have been avoided had tblz_prescriptions been built with a conservative select() statement.