appengine/datastore: add TransactionOptions.Attempts
appengine/datastore: add TransactionOptions.Attempts

Change-Id: I7737544bfef1969f220a643cced7b3f8f062b605

mzimmerman/appengine,flowlo/appengine,golang/appengine,mzimmerman/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,golang/appengine,mzimmerman/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine