If more than one place in the code submits tasks to the shared
pool, then waitTillDone() becomes unreliable.
Add a tagging mechanism, so different callsites can wait
on different sets of tasks.

Also try to protect our worker threads against exceptions from
the thread tasks code.

Change-Id: Idde664ab50008d31a2dd73910bb22f50e62ae22f
Reviewed-on: https://gerrit.libreoffice.org/27042Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>