Add grpc use_local_subchannel_pool for client (#361)

Signed-off-by: yongman <yming0221@gmail.com>

Author: yongman

tikv-client-common/src/security.rs

@@ -78,7 +78,8 @@ impl SecurityManager {
 
         let cb = ChannelBuilder::new(env)
             .keepalive_time(Duration::from_secs(10))
-            .keepalive_timeout(Duration::from_secs(3));
+            .keepalive_timeout(Duration::from_secs(3))
+            .use_local_subchannel_pool(true);
 
         let channel = if self.ca.is_empty() {
             cb.connect(&addr)