[2.8.1] CBG-1276 - Allow network= connstr override for cbdatasource DCP feeds

[bbrks]

Opened as a draft for early feedback prior to more thorough testing.

• Allows users to specify a network=... option in the connstr.
  • Default is "auto" - which uses the heuristic to pick between "default" or "external"
  • Can be set to "default" to force internal networking (no alternate address heuristic)
  • Can be set to an arbitrary network name to force alternate address for that network name (e.g. "external", or "foo")
• Default behaviour for users not specifying a network connstr option should be unchanged.
• A bit more logging around the shims.

[coveralls]

Coverage decreased (-0.04%) to 65.079% when pulling d33225c on CBG-1276 into a89e637 on release/2.8.1.