Add "Never Connected" and "Disconnected" fields

[Hibe7]

Add "Never Connected Devices" and "Disconnected Devices" stats fields.

Closes #157

[codecov[bot]]

Codecov Report

Merging #843 (c49b3ec) into master (e0fd426) will decrease coverage by 5.88%. Report is 14 commits behind head on master. The diff coverage is 0.00%.

:exclamation: Current head c49b3ec differs from pull request most recent head 721423f. Consider uploading reports for the commit 721423f to get more accurate results

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
- Coverage   67.42%   61.55%   -5.88%     
==========================================
  Files         264      184      -80     
  Lines        6429     3967    -2462     
==========================================
- Hits         4335     2442    -1893     
+ Misses       2094     1525     -569     

Files	Coverage Δ
...ib/astarte_data_updater_plant/data_updater/impl.ex	62.16% <0.00%> (ø)

... and 84 files with indirect coverage changes