Instead of creating a fresh client every single time, this creates a
single client with a single event cache store, that's cleared between
runs of the benchmark.
So the tradeoff is:
- we don't have to create a new client anymore, which means no more
async setup code, which means we can avoid using `iter_custom`; a
benefit of this is that this can be benchmarked on CI now.
- but we're also measuring the time it takes to clear the database,
which isn't trivial in itself.
Instead of having one static method duplicating an underlying
`ClientBuilder` method, we can pass the builder directly to a closure,
that will replace it. Call sites are a bit more verbose, but that would
avoid having to add duplicate `MockClientBuilder` methods for each
`ClientBuilder` method.
Benjamin Bouvier