schema: better test for issue 305

Change-Id: I791d2cd614f8cdc5921a05a0c91589f8965ed06f
index, corpus: prevent indexing dup blobs

With the sync handler + indexer in same process subscribing to all
incoming blobs, we were indexing everything twice.

Fixes camlistore.org/issue/306

Change-Id: I7da54a0e18ac613eeae36d6db29b6cdb73a37196