Resolve dependencies between etl & ndt-server

[stephen-soltesz]

Recently, several changes have altered the relationship between the etl and ndt-server repos.

• altered ndt7 archive schema such that etl/cmd/update-schema fails or ndt tables.
• disable ndt5 support in update-schema - https://github.com/m-lab/etl/pull/815
• disable etl reverse dependency in ndt-server - https://github.com/m-lab/ndt-server/pull/253

To restore support for update-schema for ndt5 without constraining the development of ndt7 or breaking etl in other ways, we should separate the NDTResult in to NDT5Result and NDT7Result.

• https://github.com/m-lab/ndt-server/issues/260

The raw tables are independent of the "unified" views we will construct from them.

[stephen-soltesz]

• NDT5Result and NDT7Result are separate and running in production.
• update-schema works correctly for ndt7

Still pending:

• ndt5 is disabled in update-schema
• ndt5 schema in production should be updated to reflect the subset of columns now used by the NDT5Result struct.