Split use_asyncio from use_smartnoise_sql and make the latter query specific

[mhauru]

Two new features:

• use_asyncio and use_smartnoise_sql are now separate configuration parameters, with the rule that both can't be true at the same time (a limitation of snsql)
• use_smartnoise_sql is no longer a global option, but set individually for each src-stats query.

Joint work with @Olajumoke01.

[Iain-S]

LGTM. Tested with MariaDB and this allowed me to use SRC_STATS as long as use-asyncio is false.

[Iain-S]

Quick thought, would it be better to have the block be

smartnoise:
  epsilon: 1
  delta: 0.1

rather than

use-smartnoise: true
epsilon: 1
delta: 0.1

since it doesn't make sense to declare epsilon and delta unless we are using smartnoise?
Leaving out (or commenting out) the whole smartnoise block would then turn it off.

[mhauru]

That makes sense, but this will soon change again when we introduce the two-stage src-stats (first a non-DP DB query, then a follow up query on the dataframe locally with DP), so I'm inclined to leave it as is now. The other PR is already in the works.