datastore: transaction: only commit once

[stephenplusplus]

I noticed an issue when trying to work with transactions. Here was my code that wasn't working:

var query = /*...*/
dataset.runInTransaction(function(transaction, done) {
  transaction.runQuery(query, function(err, entities) {
    var keys = entities.map(function(entity) {
      return entity.key;
    });
    transaction.delete(keys, function(err) {
      if (err) {/*... */}
      done();
    });
  });
}, function() {/*...*/})

This would cause:

The referenced transaction has expired or is no longer valid.

Behind the scenes, transaction.delete() made the commit API call with my delete request successfully. However, when I call done(), that runs Transaction.prototype.finalize, which will try to make another API call to commit because nothing marked my transactional operation as finalized. this.isFinalized is currently only set after a transaction.rollback() or transaction.commit(). This PR sets isFinalized = true from delete and save as well, if the request is transactional.

[stephenplusplus]

After sending this, I found some room for refactoring. But, the process will still be the same, so don't worry about waiting to review

[rakyll]

LGTM, feel free to merge.

[stephenplusplus]

My idea for refactoring quickly made testing more complicated. Will revisit after @ryanseys's API overhaul.