http_server: don't hold pool reference
http_server: don't hold pool reference

This marks the first step of a departure from the original concept:
during an asynchronous operation, the operation is not anymore
required to hold a pool reference; if a pool gets passed to the
operation, the caller needs to hold the reference.

CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy