fix: fix potential unnecessary transaction retry (#337)

A transaction could in some circumstances be retried after an abort using the previous
transaction id. This would cause the retry to abort directly as well, and then start a
new transaction. This extra loop has now been removed.

Fixes #327

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

@@ -654,7 +654,7 @@ private <T> T runInternal(final TransactionCallable<T> txCallable) {
         new Callable<T>() {
           @Override
           public T call() {
-            if (txn.isAborted()) {
+            if (attempt.get() > 0) {
               txn = session.newTransaction();
             }
             checkState(