Handle the situation in the read and send callbacks when the
httpd->readhandle has been already detached; f.e. from the previous
iteration of the read callback.
Don't detach the httpd->readcallback from the isc_httpdmgr_shutdown()
and the send callback, but rather call isc_nm_cancelread() to shutdown
the http request from the read callback.
Ondřej Surý
349b0f433b