This commit simplifies the `icingaDbOutputStage` type to contain only one
entity slice to be insert/upsert. This allows to simplify the handling in
`migrateOneType()` by removing nested loops.
Additionally, a bit of code inside that function is outsourced into a new
`utils.ChanFromSlice()` function. This makes the body of the loop over the
insert/upsert operation (the loop using the `op` variable) simple enough so
that it can just be unrolled which saves the inline struct and slice definition
for that loop.
