fix(bigtable): avoid double close on pending vrpcs

Author: mutianf

java-bigtable/google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/session/SessionPoolImpl.java

@@ -634,6 +634,11 @@ public void start(ReqT req, VRpcCallContext ctx, VRpcListener<RespT> listener) {
       this.deadlineMonitor = monitorDeadline(executorService, ctx.getOperationInfo().getDeadline());
 
       synchronized (SessionPoolImpl.this) {
+        if (isCancelled) {
+          // If the vRPC is already cancelled from a different thread return
+          // early to avoid double close
+          return;
+        }
         if (SessionPoolImpl.this.poolState != PoolState.STARTED) {
           listener.onClose(
               VRpcResult.createUncommitedError(