These helpers will work similar to the WithTx method we have added to
our dbx.DB instances, but it will use crdb.ExecuteTx or crdb.ExecuteInTx
when the backend is CockroachDB, so that transactions are retried
correctly.
Anything that uses transactions and might need to work against
CockroachDB needs to handle "RetriableError" from cockroachdb by
restarting the transaction. This will probably be a large pain if not
using these helpers or something very like them.
Subsequent changes will undertake transforming all db-transaction uses
in satellite code so that they are cockroach-safe.
Change-Id: I648b8de2168612c67b9d6eb8402bccf8286249a9
