Bump Couchbase SDK to 3.4.0.

[mikereiche]

Closes #1596.

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[mikereiche]

@programmatix
Ideally I'd use the insert() method of ReactiveAttemptTransactionContext, but I only have the CoreAttemptTransactionContext at this point and the constructor to create a ReactiveAttemptTransactionContext from a CoreAttemptTransactionContext is no public.
I've just cut-and-pasted the construction of the span, and passed that into the internal method. Note that the encoding is done with the spring-data-couchbase and note the transaction encode().