feat: add a batch get client interface which uses 'BATCH_GET' rpc for…

… read optimization (#175)

idl/rrdb.thrift

@@ -147,6 +147,29 @@ struct multi_get_response
     6:string        server;
 }
 
+struct batch_get_request {
+    1:list<full_key> keys;
+}
+
+struct full_key {
+    1:base.blob hash_key;
+    2:base.blob sort_key;
+}
+
+struct batch_get_response {
+    1:i32               error;
+    2:list<full_data>   data;
+    3:i32               app_id;
+    4:i32               partition_index;
+    6:string            server;
+}
+
+struct full_data {
+    1:base.blob hash_key;
+    2:base.blob sort_key;
+    3:base.blob value;
+}
+
 struct incr_request
 {
     1:base.blob     key;
@@ -264,6 +287,7 @@ service rrdb
     check_and_mutate_response check_and_mutate(1:check_and_mutate_request request);
     read_response get(1:base.blob key);
     multi_get_response multi_get(1:multi_get_request request);
+    batch_get_response batch_get(1:batch_get_request request);
     count_response sortkey_count(1:base.blob hash_key);
     ttl_response ttl(1:base.blob key);
     scan_response get_scanner(1:get_scanner_request request);