Ensure reader always exists for VersionedMetaData.onLoad()

This simplifies implementations of onLoad() that may want to depend on
reader not being null. Since createInMemory() resulted in a null
reader, remove that method and replace it with usages of
InMemoryRepository (which is easy enough since it was only used in
tests).

Change-Id: I8b534ef041ff7f3f54b0e744d36e99a0c8c1bb24