pkg/search: use the right method for locking the index
pkg/search: use the right method for locking the index

Since probably e93e4f3822aaa6946f19c6d2d8dd949c2edece9e we've been using
(*search.Handler).DescribeLocked instead of
(*search.Handler).Describe when serving describe queries, so the index
was never locked :(

I was so focused on debugging the recursions in describeReally that I
missed that elephant in the room.

I still think the Describe and describeReally recursions protected by
dr.wg is too complicated but that's for another CL.

Fixes #750

Change-Id: I376ee0f807df68ad1ef3752f944edc7c9bdb92ee

raff/camlistore,camlistore/camlistore,stevearm/camlistore,angelcabo/camlistore,raff/camlistore,mpl/camlistore,stevearm/camlistore,angelcabo/camlistore,raff/camlistore,angelcabo/camlistore,angelcabo/camlistore,raff/camlistore,mpl/camlistore,angelcabo/camlistore,mpl/camlistore,camlistore/camlistore,angelcabo/camlistore,mpl/camlistore,marook/camlistore,marook/camlistore,camlistore/camlistore,raff/camlistore,mpl/camlistore,mpl/camlistore,angelcabo/camlistore,mpl/camlistore,stevearm/camlistore,stevearm/camlistore,camlistore/camlistore,marook/camlistore,camlistore/camlistore,angelcabo/camlistore,marook/camlistore,marook/camlistore,marook/camlistore,camlistore/camlistore,marook/camlistore,raff/camlistore,marook/camlistore,stevearm/camlistore,raff/camlistore,angelcabo/camlistore,camlistore/camlistore,mpl/camlistore,raff/camlistore,camlistore/camlistore,stevearm/camlistore,camlistore/camlistore,stevearm/camlistore,stevearm/camlistore,stevearm/camlistore,raff/camlistore,mpl/camlistore