[CX] Decouple the degree of parallelism for scanning from the number of iodevices.

The degree of parallelism for the scan operation and all other operations up to the next exchange operator are determined by the number of configured iodevices (disks/directories used for data storage). As the default number of iodevices is 1, the degree of parallelism for a (potentially large) part of the execution plan is limited, limiting the overall performance of a query.

The problem can be alleviated by

1. configuring the correct number of iodevices or
2. simplifying (or automating) the configuration of iodevices (see MB-28048).