HTTP client connection stat callbacks are only attached to default pool

[DerGuteMoritz]

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.