Add handling of empty blocks in the RepoStore.
* Add empty block handling to repostore for put, del, has
Also added tests for all empty block handling blockstore operations. This showed there was an ambiguous identifier present for `hasBlock`, so one of the two `hasBlock` definitions was removed in `repostore`.
* Change CacheStore to RepoStore in testerasure
As CacheStore is not used in the node, update the Datastore used in the erasure coding tests to be a RepoStore. This ensures that the K > 1 cases are being tested, where they will produce empty padding blocks in the erasure-coded manifests.
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