api: nested txns

[heyitsanthony]

Certain proxies need to transactionally operate on transactions for functional correctness.

Suppose a proxy wants to guard all requests:

Txn(ctx).If(exists(proxyKey)).Then(
   OriginalTxn
).Else(
   OpGet(proxyKey)
)

However, a client issues a put/put transaction:

Txn(ctx).If(exists(x)).Then(
    OpPut("abc", "123"),
).Else(
    OpPut("def", "456"),
)

The v3 API can't handle this case because the guard txn condition can't be merged into the client transaction without changing the txn behavior. It will require an API update.

One way to add this without too much trouble is update the RequestOp and ResponseOp messages:

message RequestOp {
  // request is a union of request types accepted by a transaction.
  oneof request {
    RangeRequest request_range = 1;
    PutRequest request_put = 2; 
    DeleteRangeRequest request_delete_range = 3; 
    TxnRequest request_txn = 4;                                                                                                             
  }                                                                                                                                                           
}

message ResponseOp {
  // response is a union of response types returned by a transaction.
  oneof response { 
    RangeResponse response_range = 1;
    PutResponse response_put = 2;
    DeleteRangeResponse response_delete_range = 3;
    TxnResponse  response_txn = 4;
  }
}