to ScDocument lifetime if possible. This helps to avoid lots
of thread setup-cost while doing recalcs especially if there are
many formula-groups in the document and most of them are fairly
light-weight.

Change-Id: Idd57e1ebd0d4e492f99e31237d4a55ec9c95a121
Reviewed-on: https://gerrit.libreoffice.org/69473
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>