mpm_event: follow up to r1818804 and r1818960.

Align mod_http2 with expected returned state from process_connection hooks in
async MPMs.
When the master connection is handled, enter CONN_STATE_LINGER in any case.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1819214 13f79535-47bb-0310-9956-ffa450edef68