index-backed blockstore - synchronize acquires

[dirkmc]

Changes to the index-backed blockstore:

1. In testing we found that blockstore.Get often fails because the block cannot be retrieved from the remote store. It is very unlikely to fail because of a corrupted index. So this PR simply returns an error immediately if an operation on the blockstore returns an error (instead of trying to re-acquire the blockstore).
2. We also found that multiple concurrent calls to AcquireShard will call mount.Fetch once each. So this PR adds a sync.Once to synchronize between concurrent calls.