Add multi threading feature

[AntoJvlt]

Adding multi threading to the tool would reduce a lot the time needed to execute all the rules. As python threads run concurrently the real benefit will come when query are executed on the PostgreSQL server (maybe by using a connection pool) and when we call clustering-vt. Clustering-vt and PostgreSQL queries are the most time consuming operations when executing a rule so threads would definitively improve the performance of the tool.

It can be interesting to check if it would be better to execute a whole rule in its own thread or if threads would be spawn locally when executing PostgreSQL queries and when calling clustering-vt.

In the first case, we should make sure some operations are thread safe, for example when accessing the config or when writing/reading to files.