Speedup insert

[soad003]

The PR looks great to me. I have added the following:

• parameters to control the number of workers (--n-workers; defaults to 1 so parallel is opt-in, 0, and negative numbers can be used to as offset to cpu_count, positive values just specify the nr of workers)
• added opt-out validation to the tagpack insertion; not really related to the PR but I was startled that this was not already done
• added retry handling for deadlock situations.
• added a single thread mode that runs the insert in the main thread as before (makes pdb debugging way easier)
• added code to share the tagstore connection (one per worker process). Otherwise, every tagpack established a new connection to the database.

[soad003]

I will test some more but i think this is ready to be merged! Great work @mdragaschnig

[mdragaschnig]

Looking good.