don't set SHUTTINGDOWN until after calling the request callbacks
if we set ISC_HTTPDMGR_SHUTTINGDOWN in the http manager before calling the pending request callbacks, it can trigger an assertion.
Closes #4085 (closed)
if we set ISC_HTTPDMGR_SHUTTINGDOWN in the http manager before calling the pending request callbacks, it can trigger an assertion.
Closes #4085 (closed)