Convert AccessLogEntry class to RefCountable

Convert AccessLogEntry class to RefCountable

The AccessLogEntry objects currently are only members of the ClientHttpRequest
objects. There are cases where we need to access AccessLogEntry from server
side objects to retrieve already stored informations for the client request and use it in
server side code with format/* interface (eg use Format::Format::assemble
inside http.cc)

This patch convert AccessLogEntry class to RefCountable to allow link it with
other than the ClientHttpRequest objects.

This is a Measurement Factory project.