Finish read-write transaction when SELECT statement was aborted

[apstndb]

This PR fixed a bug that the read-write transaction doesn't finish when the execution of SELECT statement returns codes.Aborted.

Behavior

> BEGIN RW;
Query OK, 0 rows affected (0.01 sec)

(rw txn)> @{LOCK_SCANNED_RANGES=exclusive} SELECT * FROM tbl WHERE pk = 0;
Empty set (10.44 msecs)

(rw txn)> @{LOCK_SCANNED_RANGES=exclusive} SELECT * FROM tbl WHERE pk = 0;
ERROR: spanner: code = "Aborted", desc = "Transaction was aborted."
>  

This PR doesn't change the behavior of codes other than codes.Aborted so syntax error doesn't abort transaction. It seems user friendly.

(rw txn)> SELECT * FROM tbl WHERE pk = '';
ERROR: spanner: code = "InvalidArgument", desc = "No matching signature for operator = for argument types: INT64, STRING. Supported signature: ANY = ANY [at 1:25]\\nSELECT * FROM tbl WHERE pk = \\'\\'\\n                        ^"
(rw txn)> 

closes #95

[yfuruyama]

LGTM. Thanks!