Realm Management: port to Xandra db driver

[Annopaolo]

In the spirit of astarte-platform/astarte_data_access#60, replace the old and unmantained CQEx driver with Xandra.

[codecov]

Codecov Report

Merging #771 (4e876ab) into release-1.1 (e0fd426) will increase coverage by 1.47%.
Report is 12 commits behind head on release-1.1.
The diff coverage is 70.60%.

❗ Current head 4e876ab differs from pull request most recent head b6966d9. Consider uploading reports for the commit b6966d9 to get more accurate results

@@               Coverage Diff               @@
##           release-1.1     #771      +/-   ##
===============================================
+ Coverage        67.42%   68.90%   +1.47%     
===============================================
  Files              264      259       -5     
  Lines             6429     5994     -435     
===============================================
- Hits              4335     4130     -205     
+ Misses            2094     1864     -230     

Files Changed	Coverage Δ
...anagement/lib/astarte_realm_management/c_system.ex	72.72% <0.00%> (-1.19%)	⬇️
..._management/lib/astarte_realm_management/config.ex	50.00% <ø> (-12.50%)	⬇️
...management/lib/astarte_realm_management/queries.ex	70.10% <69.30%> (+2.89%)	⬆️
..._management/lib/astarte_realm_management/engine.ex	82.94% <79.62%> (+4.48%)	⬆️
...e_realm_management/lib/astarte_realm_management.ex	100.00% <100.00%> (ø)

... and 124 files with indirect coverage changes