## Problem
We recently added slow GetPage request logging. However, this
unintentionally included the flush time when logging (which we already
have separate logging for). It also logs at WARN level, which is a bit
aggressive since we see this fire quite frequently.
Follows https://github.com/neondatabase/neon/pull/10906.
## Summary of changes
* Only log the request execution time, not the flush time.
* Extract a `pagestream_dispatch_batched_message()` helper.
* Rename `warn_slow()` to `log_slow()` and downgrade to INFO.
## Problem
We don't have good observability for "stuck" getpage requests.
Resolves https://github.com/neondatabase/cloud/issues/23808.
## Summary of changes
Log a periodic warning (every 30 seconds) if GetPage request execution
is slow to complete, to aid in debugging stuck GetPage requests.
This does not cover response flushing (we have separate logging for
that), nor reading the request from the socket and batching it (expected
to be insignificant and not straightforward to handle with the current
protocol).
This costs 95 nanoseconds on the happy path when awaiting a
`tokio::task::yield_now()`:
```
warn_slow/enabled=false time: [45.716 ns 46.116 ns 46.687 ns]
warn_slow/enabled=true time: [141.53 ns 141.83 ns 142.18 ns]
```
