Add pkg/index/kvfile: index.Storage impl on top of github.com/cznic/kv

Change-Id: I4adb44c4e70ad8e68b4292fefbbd02e0a52beb6c
camlistored: link in kvfile

Change-Id: I55f65275b45f7de0efc0c52e84ac61f4baebaa38
devcam: make kvfile the default dev-server indexer.

Memory index requires -memindex now.

Change-Id: I6df424e88c3783e397c528cd8633402a9edf3d95