Refactor the rpc clients.

There are no API changes here, the goal is just to simplify the code.

Signed-off-by: Nick Cameron <nrc@ncameron.org>

Author: nrc

Cargo.toml

@@ -19,6 +19,7 @@ name = "tikv_client"
 
 [dependencies]
 regex = "1"
+derive-new = "0.5"
 failure = "0.1"
 futures-preview = { version = "0.3.0-alpha.15", features = ["compat"] }
 grpcio = { version = "0.5.0-alpha", features = [ "secure", "prost-codec" ], default-features = false }