feat(adapter/kv): support async iterating on scan results

[PragmaTwice]

Which issue does this PR close?

Closes #3639.

Rationale for this change

Instead of performing a full scanning and returning a Vec<String>, now we introduce an associated type into kv::Adapter:

type Scanner: Scan;

So that the implementation can do paged scanning and returns a ScanIter, and users can iterate over it to get keys in batches from the server.

What changes are included in this PR?

• add associated type ScanIter into kv::Adapter and refactor KvLister and Adaptor's Accessor impl;
• refactor all services using kv::Adapter, especially for which has list cap;
• for sqlite (via sqlx): now, instead of using fetch_all, we can use fetch to fetch data in batches instead of fetching all data.

Are there any user-facing changes?

No changes for end users.

[PragmaTwice]

The lifetime handling here is a bit tricky.

sqlx::query_scalar() and its .fetch() heavily depend on lifetime parameter restrictions:

pub fn fetch<'e, 'c: 'e, E>(self, executor: E) -> BoxStream<'e, Result<O, Error>>
    where
        'q: 'e,
        E: 'e + Executor<'c, Database = DB>,
        DB: 'e,
        A: 'e,
        O: 'e,

So here to extend the lifetime (since we don't use GAT in our API here e.g. type ScanIter<'a>, in order to be compatible to trait List which is also not a GAT in Access.), we need to carry both the Pool and also the query &str along our Stream into return value and put it into a self-referencing structure and then we don't need to care the lifetime checking.

[Xuanwo]

Thank you, @PragmaTwice, for this excellent pull request!

[Xuanwo]

There are some critical bug fixes that need to be released. I will merge this after we create a new release.