Support `upsert` migrations

[gggeek]

It is currently possible to achieve an upsert (insert-or-update) via a 3-step process:

• step 1: content/load, match via ..., set reference to count
• step 2: content/create, if count == 0
• step 3: content/update, if count == 1, with same match conditions as load

It would be nice to be able to do the same in a single migration step.

Besides contents, upsert could be used for most (all?) other Repo Entities.

NB: there are subtleties involved, such as: what if the pre-existing content has values for fields not specified in the upsert? Or extra locations? Same applies to ContentType defs...

[gggeek]

A document describing many of the subtleties of upserts for Contents has been added, will be in upcoming release 6.1