RFC: etcdserver: check permission in Txn() RPC

[mitake]

This PR adds permission checking in Txn() RPC. Currently, the check works like below:

1. check compare requests and return permission denied error if the requests has denied range operations
2. check one of the request sequences for success and failure cases and return permission denied error if the requests has denied operations

It means that the transaction request can be completed even if the request sequences that aren't executed has invalid permission. I think it is ok but how do you think? @xiang90 @heyitsanthony

[xiang90]

s.applyV3.Txn(ctx context.Context, username string, r.Txn)

[mitake]

@heyitsanthony the idea of authApplierV3 seems to be elegant but adding a new argument of auth would make it complex (maybe I'm missing something?). Like @xiang90 said, adding a single function for permission checking and call it like other existing permission checking functions (e.g. IsPutPermitted()) would be the simplest design. How do you think?