Support importing individual blocks from a folder

[Arkenan]

Hive gives a client initial blocks either via a chain.rlp file or individual block files in a /blocks folder. We already support the first case via the import flag. However, we just skip the latter. We would need to support that to make the consensus suite be able to at least run the tests correctly.

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For more context on consensus suite problems, our node logs:

Started store engine
Storing initial state from genesis
Storing genesis block with number 0 and hash 0x1bd8…e792

But then the simulator shows:

last block hash mismatch:
  want 0xca2e5143a2384414736a32c898f032d835feb43dec9cd6a77f7e2b2695e4cf14
   got 0x1bd8cc1b182ec860a1042ad2e1c130fd5e56cb050b9f1fe368de36725cf2e792

Genesis is correctly imported, but it is considered latest, which is not.