api.Client: support isolated read-after-write

[benashz]

This PR adds the ability for a Client to track all cluster replication states. The new ReplicationStateStore provides response and request callback methods to the Client, and can be used across Client clones.

import  (
        "log"

        "github.com/hashicorp/vault/api"
)

func main() {                        
        config := api.DefaultConfig()  
        config.ReadYourWrites = true     
        client, err := api.NewClient(config)
        if err != nil {                           
                log.Fatal(err)
        }

        [...]    
}

Related PR: hashicorp/terraform-provider-vault#1188

[ncabatoff]

I'm going to approve because the only thing still being worked out is the name for the config option, and I'm happy to go with whatever Brian and Ben agree on.