This takes the loaing time from 1m38 to 15s.

Speed up finding existing pool items in the pool by storing a
sorted_vector, sorted by a compare operator on the item.

memcmp is faster than operator< on std::vector because operator< seems
to be trying to use some kind of lexicographical compare

Change-Id: Id72527106d604adb8cd2d158bb42f89e2b16a85d
Reviewed-on: https://gerrit.libreoffice.org/71009
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>