Fully support mariadb

[Iain-S]

In principle, we should be able to support a wide array of db management systems but we do a few things (the schema change, in particular) which may not work cross-platform.

[Iain-S]

#110 adds the basics to be able to support MariaDB but leaves MariaDB users unable to use src-stats. This is because:

1. smartnoise SQL doesn't support mariadb (but it does support pandas so I wonder whether we could hack it in)
2. we use asyncpg if smartnoise SQL is turned off and asyncpg is postgres specific (I presume)

Possible solutions are

1. decouple use_asyncio from use_smartnoisesql whereas we currently do use_asyncio = not use_smartnoisesql
2. use https://pypi.org/project/aiomysql/ for async mariadb support

[Iain-S]

See also #118

[Iain-S]

We now support source stats and even snsql queries so I'll consider this done.