HTTP client connection stat callbacks are only attached to default pool

# Problem

When creating a custom connection pool via `aleph.http/connection-pool`, it doesn't have the stat callbacks registered via `register-connection-stats-callback` attached to it. There's also no (clean) way to manually attach them because the `aleph.http/connection-stats-callbacks` var is private.

# Solution

Instead of only attaching the stat callbacks to `default-connection-pool`, make them the default value for the `:stats-callback` option of `aleph.http/connection-pool`.

Start at aleph.http/connection-pool and compare its :stats-callback default with default-connection-pool and register-connection-stats-callback. Trace how aleph.http/connection-stats-callbacks is used. Done means custom connection pools receive the registered callbacks by default, matching the stated behavior.

Written by the indexing model from the issue text.