mpm_event: Follow up to r1918482: CONN_STATE_LINGER* are not the last anymore.

mpm_event: Follow up to r1918482: CONN_STATE_LINGER* are not the last anymore.

Since CONN_STATE_ASYNC_WAITIO, we cannot check for < or >= CONN_STATE_LINGER
anymore to determine if in an lingering close state, so let's add a new
CONN_STATE_IS_LINGERING_CLOSE() macro for this and use it in mpm_event.

The test for state == CONN_STATE_LINGER in process_lingering_close() is a
bit weak too in order to call ap_start_lingering_close() the first time only,
so have a conn_state->linger_started flag instead.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918491 13f79535-47bb-0310-9956-ffa450edef68