mpm_event,mod_http2,mod_status: Follow up to r1918257: CONN_STATE_ASYNC_WAITIO.

mpm_event,mod_http2,mod_status: Follow up to r1918257: CONN_STATE_ASYNC_WAITIO.

Per discussion on PR #449, have a separate state for returning the connection
to the MPM to wait for an IO (namely CONN_STATE_ASYNC_WAITIO), rather than
(ab)using CONN_STATE_PROCESSING.

This removes the need for AGAIN added in r1918257 (for now), and AP_MPMQ_CAN_AGAIN
is renamed to AP_MPMQ_CAN_WAITIO.

This is also the state that mod_status accounts for, so rename ->processing
to ->wait_io in process_score (shows as "wait-io" in mod_status and mod_lua).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918482 13f79535-47bb-0310-9956-ffa450edef68