PROPFIND 'propname' is the special usage to retrieve all the
properties available on the URI resource, their names only.

See <https://tools.ietf.org/html/rfc4918#section-9.1> for
PROPFIND 'propname' definition.

Add cache usage in Content::getProperties as well.
The caching model is simple: a simple lifetime limit of 10 seconds
to declare the property name list stale and request another list,
accessing the Net.

This should reduce the number of PROPFIND calls on the Net.

Change-Id: Ie4ebd946dd81583dc964a62c7744f3e2c716c737
Reviewed-on: https://gerrit.libreoffice.org/28273Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Giuseppe Castagno <giuseppe.castagno@acca-esse.eu>