Couchbase transactions example needs to be on 3.0.0-RC2 as 3.0.0 has …

[mikereiche]

…reactor with a regression.

Also includes some improvements to sample.

Closes #648.

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[mp911de]

This pull request introduces changes everywhere with a description that doesn't reflect the actual change. We're upgrading to a newer Spring Boot version anyway. If you want to contribute a transactional example, then please submit a pull request introducing only that without touching all possible bits.