rgw/beast: reference count Connections for timeout_handler

rgw/beast: reference count Connections for timeout_handler

resolves a use-after-free in the timeout_handler, where a timeout fires
and schedules the timeout_handler for execution, but the coroutine exits
and destroys the socket before asio executes the timeout_handler

timeout_handler now holds a reference on the Connection to extend its
lifetime

now that the Connection is allocated on the heap, we can include the
parse_buffer in this memory instead of allocating it separately

Signed-off-by: Casey Bodley <cbodley@redhat.com>