store: refactor `queries.rs` and `types.rs`

[drahnr]

The queries.rs file has grown to 1.5k loc which is far too many. There are also types that are oftentimes only relevant to a single query or insert/upsert that accumulated in types.rs.

The goal is to find a more coherent way of rerpresenting the queries, where all relevant types "live together".

[Mirko-von-Leipzig]

I've previously had success with a custom DbTx type wrapper, and then having the queries in submodules/files as appropriate with

impl DbTx {
     fn query_a(&mut self) {...}
}

This also let's one easily add tests to the same file/module if needed. Often isn't required, but sometimes its good to check constraints.

ymmv though; don't feel like the above is a requirement in any way.

[drahnr]

The immediate goals are:

• split the queries.rs into multiple files - done in refactor: split queries into files #1148
• unify type naming for insertions and queries
• investigate if we can separate internal-only functions and avoid any conversions of the user side
• consider introducing struct Connection with implementation for all valid queries

[drahnr]

I am still unsure how much we'd gain from, since we're already have a wrapping layer.

impl DbTx {
     fn query_a(&mut self) {...}
}

Maybe not at the queries level but the level above, this might make more sense.