du sql query: eliminate use of count(a.*)

[andrewbogott]

The particular syntax count(a.*) in this query doesn't parse on mariadb (or, presumably, mysql). Counting a.uid is equivalent; we can be confident that there aren't any rows without uid set because the query includes 'a.uid is not NULL'.

Addresses https://github.com/wamdam/backy2/issues/71

Here are examples in mariadb:

mysql:galera_backup@localhost [eqiad1_ceph_backy]> select a.uid, a.size, count(a.) from blocks a where a.version_uid="bc7eae94-d8f8-11ea-8823-f0921c05f570" and a.uid is not NULL; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near ') from blocks a where a.version_uid="bc7eae94-d8f8-11ea-8823-f0921c05f570" and ' at line 1

mysql:galera_backup@localhost [eqiad1_ceph_backy]> select a.uid, a.size, count(a.uid) own_shared, (select count(*) cnt from blocks where uid=a.uid) shared from blocks a where a.version_uid="bc7eae94-d8f8-11ea-8823-f0921c05f570" and a.uid is not NULL group by a.uid, a.size limit 2; +----------------------------------+---------+------------+--------+ | uid | size | own_shared | shared | +----------------------------------+---------+------------+--------+ | 000f3a5685vDGo3bcSJEXE4i34ioQHAd | 4194304 | 1 | 4 | | 009da580dfystrfRKiYZMEBX4dFbbSw6 | 4194304 | 1 | 4 | +----------------------------------+---------+------------+--------+ 2 rows in set (0.033 sec)