speedup for propfinds on shared folders

A PROPFIND request still takes ~10 times longer on a shared directory than on an unshared one, but without these patches it takes about ~200 times as long. These changes seem to work for me.

Before: ~70 seconds, after: ~4 seconds. Unshared access: ~0.3 seconds. 

This is a significant improvement and people ask for it: see http://mail.kde.org/pipermail/owncloud/2013-June/009758.html, ff, and issue #2846 .

@icewind1991 @MTGap Any objections? Are additional patches needed? 

@karlitschek Please consider this for 5.0.8 (RC2?)