pkg/index: check for deletions in EdgesTo

Also introduced and used kvEdgeBackward

http://camlistore.org/issue/191

Change-Id: Ifdca9f2453ec0c7db55afc40cf0b5f1441b83158
Merge "pkg/index: check for deletions in EdgesTo"
serverconfig: add 'memoryIndex' bool option to high-level config

Change-Id: If5edb272858f186931eeb57579e221bde3efb07c