Support optimizer statistics

[yfuruyama]

This PR has two changes to support optimizer statistics:

1. Support ALTER STATISTICS statement.
2. Show optimizer statistics package name in verbose mode.

Example:

spanner> ALTER STATISTICS auto_20210829_05_22_28UTC SET OPTIONS (allow_gc = false);
Query OK, 0 rows affected (40.93 sec)

spanner> SELECT * FROM Singers;
+----------+-----------+----------+------------+------------+
| SingerId | FirstName | LastName | SingerInfo | BirthDate  |
+----------+-----------+----------+------------+------------+
| 1        | b         | Richards | NULL       | 1970-09-03 |
| 2        | Catalina  | Smith    | NULL       | 1990-08-17 |
| 3        | Alice     | Trentor  | NULL       | 1991-10-02 |
| 4        | Lea       | Martin   | NULL       | 1991-11-09 |
| 5        | David     | Lomond   | NULL       | 1977-01-29 |
+----------+-----------+----------+------------+------------+
5 rows in set (2.75 msecs)
timestamp:            2021-08-31T13:58:59.509876+09:00
cpu time:             0.9 msecs
rows scanned:         5 rows
deleted rows scanned: 0 rows
optimizer version:    2
optimizer statistics: auto_20210829_05_22_28UTC

[yfuruyama]

@apstndb Could you review this PR when you have a chance? I want to hear feedback about if showing optimizer statistics package name in verbose mode makes sense to you.

[apstndb]

I want to hear feedback about if showing optimizer statistics package name in verbose mode makes sense to you.

"optimizer statistics" seems not ambiguous and reasonably short.
LGTM.

[apstndb]

Alignment test 👀

[yfuruyama]

Thanks for reviewing!

"optimizer statistics" seems not ambiguous and reasonably short.

Yeah, I wondered if we should use optimizer statistics package or optimizer statistics, but the former one looks little bit long to me, so I chose optimizer statistics.