rpc: Refactor rpc/lib

root `rpc` package should be renamed to `jsonrpc`
`rpcclient` package should be renamed to `client`
`rpcserver` package should be renamed to `server`

new API:

```
import "github.com/tendermint/tendermint/rpc/lib/jsonrpc/client"

c := client.New()
c2 := client.NewURI()
```