provide better API client for debuggin MVCC info

[tiancaiamao]

Currently, the txn related API is easy to use:

txn := store.Begin()
txn.Get()
txn.Set()
txn.Scan()
....

snap := store.GetSnapshot()
snap.Get()
snap.Scan()
...

But the others are not easy to use.

The coprocessor API is implemented by the cop client:
https://github.com/pingcap/tidb/tree/master/pkg/store/copr

And the MVCC debug API is here:
https://github.com/pingcap/tidb/tree/master/pkg/store/helper

Both are in the tidb repo.

The coprocessor client API is defined here:
https://github.com/pingcap/tidb/blob/e98cf3d4cad24e8152574b4f201f943aab557d0d/pkg/kv/kv.go#L316-L318

Considering that sending coprocessor request require the DAG (which is done by the planner), it's hard to construct the DAG request manually, it's reasonable to leave coprocessor API to the tidb repo.

But I think we can move MVCC debug API here to client-go.