Replace usage of DBClient reactive to blocking in OCI examples

[spericas]

In 3.x the ATP OCI example had this:

createDbClient(walletContent)
        .flatMap(dbClient -> dbClient.execute(exec -> exec.query("SELECT 'Hello world!!' FROM DUAL")))
        .first()
        .map(dbRow -> dbRow.column(1).as(String.class))
        .ifEmpty(() -> res.status(404).send())
        .onError(res::send)
        .forSingle(res::send);

Requires #6991

However, since #6991 is in progress, let's update the usage to use .await() to block.

[spericas]

This one relates to other OCI example issues (#6872 and #6873).

@aserkes I'll assign this one to you as it relates to the others. Feel free to assign it back to me if after moving examples to Nima, we still need to replace DbClient.

[romain-grecourt]

We've decided to update DBClient to blocking, thus we should not update all usages of DBClient. In the mean time we can use .await() on the Single and Multi instead of using JDBC directly.

[spericas]

@romain-grecourt Roger, title updated.

[romain-grecourt]

Fixed in #6859