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Environment

• VerneMQ

In light of the gradual demise of CQEx in favor of Xandra, astarte_pairing should be adapted to use Xandra instead of CQEx.

This mainly translates to:

• Add Xandra as a dependency
• Start the Xandra connection
• Translate all queries from CQEx to Xandra, both in lib and in test.

To see an example of a service already using Xandra, have a look at astarte_housekeeping (and this PR h