[perf] Add caching on `dashboard_load_id`; reusable MetadataProviders (#45050)

**MetadataProvider cache**

The reusable MetadataProviders is a generally useful thing - within some
dynamic scope like an API request, a `clojure.core.cache` instance can
be provided as `lib.metadata.jvm/*metadata-provider-cache*`, and that
will be used to cache the provider for each `database-id`.

Reusing those providers can significantly reduce appdb traffic,
re-fetching the same metadata in different `MetadataProvider`
instances.

**Dashboard loads**

The FE can supply `?dashboard_load_id=uuid_string` on requests for a
dashboard load:

- `/api/dashboard/:id`
- `/api/dashboard/:id/query_metadata`
- `/api/dashboard/:id/dashcard/:dashcard_id/card/:card_id/query`

That provides a good cache key across that burst of N+2 API requests
that are part of a single load. Currently it is used for two things:

- Caching `get-dashboard` (used by the first two requests)
- Selecting the `clojure.core.cache` for `*metadata-provider-cache*`

Both use a TTL of 10 seconds, which is generally enough for the first
screenful of a dashboard load, if not the whole thing. (Browsers will
only issue a handful of requests at once, so a few long-running queries
might delay later parts of the dashboard from loading until after the
TTL expires.)