Print mutation count in verbose mode

[apstndb]

Cloud Spanner limit mutation count in a commit. It is hard to calculate and not well documented about complex situations.

• spanner: Programmatically Calculating Spanner Max Mutation Set googleapis/google-cloud-go#1721

google-cloud-go spanner/v1.14.0 support CommitStats which contains mutation count so spanner-cli can print it.

$ spanner-cli -v 
> INSERT tbl(pk) VALUES(1);
Query OK, 1 rows affected (0.95 sec)
timestamp:      2021-02-09T14:03:22.980381+09:00
mutation_count: 1

> BEGIN RW;
Query OK, 0 rows affected (0.04 sec)

(rw txn)> INSERT tbl(pk, col) VALUES(2, 0);
Query OK, 1 rows affected (0.53 sec)

(rw txn)> COMMIT;
Query OK, 0 rows affected (0.14 sec)
timestamp:      2021-02-09T14:03:58.550281+09:00
mutation_count: 2

[yfuruyama]

Thanks for contribution. LGTM!