Improve tikv-dump to support mvcc decoding.

[disksing]

In short, I propose to add a new flag 'mvcc' to dump mvcc encoded key-value pairs. All cf's key-values are encoded as following:

	default	lock	write
key	z{encoded_key}{start_ts(desc)}	z{encoded_key}	z{encoded_key}{commit_ts(desc)}
value	{value}	{flag}{primary_key}{start_ts(varint)}	{flag}{start_ts(varint)}

• Leading z is created by raftstore.
• encoded_key is a key received from client after been encoded by encodeBytes. https://github.com/pingcap/tikv/blob/master/src/util/codec/bytes.rs#L31
• value is the value received from client.
• ts(desc) is a txn's ts after been encoded by encode_u64_desc. https://github.com/pingcap/tikv/blob/master/src/util/codec/number.rs#L76
• flag a byte could be either P(PUT), D(DELETE), L(LOCK), or R(Rollback).
• primary is a txn's primary key after been encoded by encode_compact_bytes. https://github.com/pingcap/tikv/blob/master/src/util/codec/bytes.rs#L56
• ts(varint) is a txn's ts after been encoded by encode_var_u64. https://github.com/pingcap/tikv/blob/master/src/util/codec/number.rs#L92

Usage example (dump a key's write cf):

./tikv-dump --store "foo/db" -mvcc -k "t\\200\\000\\000\\000\\000\\000\\000\\377R_r\\200\\000\\000\\000\\000\\377\\004\\251\\322\\000\\000\\000\\000\\000\\372" -c write

Output (2 PUT records found):

key: zt\\200\\000\\000\\000\\000\\000\\000\\377R_r\\200\\000\\000\\000\\000\\377\\004`B\\000\\000\\000\\000\\000\\372\\372\\243\'W\\350{\\377\\355
value: P\\227\\200\\220\\212\\201\\225\\266\\256\\005
type: PUT
commit_ts: 386421884507652114 
start_ts: 386421884402794519

key: zt\\200\\000\\000\\000\\000\\000\\000\\377R_r\\200\\000\\000\\000\\000\\377\\004`B\\000\\000\\000\\000\\000\\372\\372\\243\'\\303\\n\\323\\377\\351
value: P\\212\\200\\260\\367\\316\\207\\266\\256\\005
type: PUT
commit_ts: 386421424369958934
start_ts: 386421424265101322

[siddontang]

/cc @mrmiywj

[BusyJay]

I think subcommand is better in this case.

[siddontang]

yes, may we should use subcommand to distinguish mvcc and raft.

[mrmiywj]

Thx! I'll take a look at it.