txn feature request: tree-unmodified

### `consul version` for both Client and Server
Client: 7.5
Server: 7.5

### client and server info dumps
(not relevant - feature request)

### Description of the Issue (and unexpected/desired result)

A txn has the ability to return an entire prefix-tree of keys.

A prefix-based fetch returns a Modify-Index header with the maximum index of any subkey.

I would like the ability to assert (without altering the tree) that a key prefix tree has not been modified, by supplying that Modify-Index.

(This is a nice-to-have: I've a bunch of operations to run against consul and this ability would vastly increase the ability to simplify my txn code.)