sqlx does not allow setting persistence of QueryAs, QueryScalar

[philippeitis]

sqlx provides functionality for setting the persistence of Query, but not QueryAs, QueryScalar.

In my case, I'm using caching to store parsed data from SQLite, and I only load data which is not in the cache, using WHERE ... IN (uncached_ids). However, each of these statements is very likely to be unique, which causes sqlx to cache a lot of similar statements and bloat memory usage in my application.

[abonander]

This should be a pretty easy fix, just copy the method from Query: https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/query.rs#L100

QueryAs wraps Query and QueryScalar wraps QueryAs so the implementation for the new methods would just be self.inner.persistent(value);

Ignore the persistent() method in the Execute impls, that's the getter for the set value and it's already implemented correctly for all three types; we should probably rename that to is_persistent() to be _con_sistent with Rust idioms but that can be done another time.