Preserving order on multiple inserts

[mmanciop]

What are the guarantees in terms of insertion order for R2dbcRepository#saveAll for new items? Some experiments I have been playing around with seem to show that the insertion order is not honored, which makes sense since SimpleR2dbcRepository#saveAll is just a flatMap built on SimpleR2dbcRepository#save, and the latter queues an asynchronous job on the TcpResources pool of Reactor-Netty.

While our-of-order updates is pretty much a non-issue in my experience, not honoring the insertion order is very confusing when the database is generating sequential primary keys.

For reference, #7 enhances the tests to show sporadic failures due to the insertion order not been deterministic.

[mp911de]

That's fixed now via #12.