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

OStorageImpl is spending time iterating over its m_aChildrenVector to
find stuff by name, so just use a std::unordered_map.
Also return iterator from FindElement, so we can avoid searching the map
twice.
This was probably the biggest win.

Change-Id: I30776bad5377d14144fc7a47e86527e2cdb62a83
Reviewed-on: https://gerrit.libreoffice.org/70313
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>