RecordAccumulator builds a set of datastore Entity records from a set of
leveldb logfiles in a directory tree (which is how we receive them for
database backup/restore testing).
This CL also refactors some of the logfile test code out of
LevelDbLogReaderTest so that we can reuse it for building test logs.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167313553
