Support build & parse RawKV keys

[pingyu]

Close #14

Preview readme.md.

Unit tests:

go test -timeout 30s github.com/disksing/mok

ok  	github.com/disksing/mok	0.484s	coverage: 42.2% of statements

Manual tests:

❯ go build
❯ ./mok 7200000074657374ff0000000000000000f7
"7200000074657374ff0000000000000000f7"
└─## decode hex key
  └─"r\000\000\000test\377\000\000\000\000\000\000\000\000\367"
    ├─## decode mvcc key
    │ └─"r\000\000\000test"
    │   └─## decode keyspace
    │     ├─key mode: rawkv
    │     ├─keyspace: 0
    │     └─"test"
    └─## decode keyspace
      ├─key mode: rawkv
      ├─keyspace: 0
      └─"test\377\000\000\000\000\000\000\000\000\367"
❯ ./mok --keyspace-id 0 --key-mode rawkv --raw-key test
built key: 7200000074657374FF0000000000000000F7

Manual regression tests:

❯ ./mok 7A7480000000000007FF8F5F728000000000FF083BBA0000000000FAFA6C400A6673FFFE
"7A7480000000000007FF8F5F728000000000FF083BBA0000000000FAFA6C400A6673FFFE"
└─## decode hex key
  └─"zt\200\000\000\000\000\000\007\377\217_r\200\000\000\000\000\377\010;\272\000\000\000\000\000\372\372l@\nfs\377\376"
    └─## decode rocksdb data key
      └─"t\200\000\000\000\000\000\007\377\217_r\200\000\000\000\000\377\010;\272\000\000\000\000\000\372\372l@\nfs\377\376"
        ├─## decode mvcc key
        │ ├─"t\200\000\000\000\000\000\007\217_r\200\000\000\000\000\010;\272"
        │ │ ├─## table prefix
        │ │ │ └─table: 1935
        │ │ └─## table row key
        │ │   ├─table: 1935
        │ │   └─row: 539578
        │ └─ts: 401875853330087937 (2018-07-31 18:58:38.819 +0800 CST)
        └─## table prefix
          └─table: 2047
❯ ./mok --table-id 43 --row-value 81934
built key: 7480000000000000FF2B5F728000000000FF01400E0000000000FA
❯ ./mok --table-id 43 --index-id 50 --index-value 81934
built key: 7480000000000000FF2B5F698000000000FF0000328000000000FF01400E0000000000FA
❯ ./mok --keyspace-id 255 --table-id 43 --row-value 81934
built key: 780000FF74800000FF000000002B5F7280FF0000000001400E00FE