cmd: add dump command to dump metadata to a file and load to load from a file

[SandyXSD]

close #50

Todo:

• calculate slice refs when loading
• customize json marshal to stream encode/decode; and to make the dumped file more compact

[davies]

@Sandy4999 Could you add a few unit tests for them? loading a sample file into redis/db, verify some functions, then dump it again, and compare the results.

[davies]

pending on unit test