fix: retry non-idempotent long-running RPCs (#141)

RPCs returning a long-running operation, such as CreateDatabase, CreateBackup and RestoreDatabase, are non-idempotent and cannot be retried automatically by gax. This means that these RPCs sometimes fail with transient errors, such as UNAVAILABLE or DEADLINE_EXCEEDED. This change introduces automatic retries of these RPCs using the following logic:
1. Execute the RPC and wait for the operation to be returned.
2. If a transient error occurs while waiting for the operation, the
   client library queries the backend for the corresponding operation.
   If the operation is found, the resumes the tracking of the existing
   operation and returns that to the user.
3. If no corresponding operation is found in step 2, the client library
   retries the RPC from step 1.

Fixes GoogleCloudPlatform/java-docs-samples#2571

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -21,6 +21,7 @@
 import com.google.api.gax.longrunning.OperationSnapshot;
 import com.google.api.gax.longrunning.OperationTimedPollAlgorithm;
 import com.google.api.gax.retrying.RetrySettings;
+import com.google.api.gax.rpc.StatusCode;
 import com.google.api.gax.rpc.TransportChannelProvider;
 import com.google.api.gax.rpc.UnaryCallSettings;
 import com.google.cloud.NoCredentials;
@@ -291,7 +292,8 @@ private Builder() {
           .setRetrySettings(longRunningRetrySettings);
       databaseAdminStubSettingsBuilder
           .updateBackupSettings()
-          .setRetrySettings(longRunningRetrySettings);
+          .setRetrySettings(longRunningRetrySettings)
+          .setRetryableCodes(StatusCode.Code.DEADLINE_EXCEEDED, StatusCode.Code.UNAVAILABLE);
     }
 
     Builder(SpannerOptions options) {
@@ -581,6 +583,7 @@ public Builder setEmulatorHost(String emulatorHost) {
       return this;
     }
 
+    @SuppressWarnings("rawtypes")
     @Override
     public SpannerOptions build() {
       // Set the host of emulator has been set.