Individually, these don't make much difference, but they add up
to a halving the time to save on my machine.

fileaccess::TaskManager was spending lots of time iterating over it's
internal data, so flatten the structures here, to avoid cache-unfriendly
pointer chasing.

Change-Id: Ic1e2e3958d10f3266aad5ddbcbd68ef1c7285dc7
Reviewed-on: https://gerrit.libreoffice.org/70312
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>