Only remove init connections to the database on server shutdown.

Summary:
With MariaDB 10.2 libmysqlclient was replaced with libmariadb that
changed how establishing database connections behaves. The MySQL
QSQL driver calls mysql_server_end() on QSqlDatabase::removeDatabase()
if the overall connection count dropped to 0 (which it does when
the init connection is removed).
A future QSqlDatabase:addDatabase() would call mysql_server_init()
again, but this no longer works with libmariadb as that one only
allows calling mysql_server_init() once. Future calls are simply
ignored.

In order to prevent this from happening we have to keep the
init connection open until the server shuts down, so the connection
count only drops to 0 at shutdown and mysql_server_end() isn't
called before.

This is a workaround for QTBUG-63108

CCBUG: 383991

Reviewers: dvratil, mlaurent

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7858