mpm_event,mod_http2: Keep compatibility with CONN_STATE_PROCESSING + OK

mpm_event,mod_http2: Keep compatibility with CONN_STATE_PROCESSING + OK

Before r1918022, returning OK with CONN_STATE_PROCESSING to mpm_event was
handled like/by CONN_STATE_LINGER "to not break old or third-party modules
which might return OK w/o touching the state and expect lingering close,
like with worker or prefork MPMs".

So we need a new return code to be allowed to apply the new POLLIN/POLLOUT
behaviour for CONN_STATE_PROCESSING, thus revive AGAIN as introduced by
Graham some times ago for a nonblocking WIP (moved to a branch/PR since then).

MPM event will advertise its ability to handle CONN_STATE_PROCESSING + AGAIN
with AP_MPMQ_CAN_AGAIN, and mod_http2 can use that to know how to return to
the MPM as expected. When !AP_MPMQ_CAN_AGAIN modules/mod_http2 can still use
CONN_STATE_WRITE_COMPLETION + CONN_SENSE_WANT_READ + c->clogging_input_filters
which will work in mpm_even-2.4.x still.

* include/ap_mmn.h:
  Bump MMN minor for AP_MPMQ_CAN_AGAIN and AGAIN.

* include/ap_mpm.h:
  Define AP_MPMQ_CAN_AGAIN.

* include/httpd.h:
  Define AGAIN.

* modules/http2/h2.h:
  No need for H2_USE_STATE_PROCESSING anymore with AP_MPMQ_CAN_AGAIN.

* modules/http2/h2_c1.c:
  For !keepalive case return to the MPM using CONN_STATE_PROCESSING + AGAIN
  or CONN_STATE_WRITE_COMPLETION + c->clogging_input_filters depending on
  AP_MPMQ_CAN_AGAIN only.

* modules/http2/h2_session.c:
  Can return to the MPM for h2_send_flow_blocked() provided it's async only.

* server/mpm/event/event.c:
  Rework process_socket()'s CONN_STATE_PROCESSING to handle AGAIN and preserve
  compatibility. Have a lingering_close label to goto there faster when
  process_lingering_close() is to be called. Improve relevant comments.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918257 13f79535-47bb-0310-9956-ffa450edef68