core: follow up to r1839997: some runtime optimizations.

We don't mind about cleaning up a connection filter when its pool is being
cleaned up already. For request filters, let pending_filter_cleanup() do
nothing if the given filter is not pending (anymore), which allows to save a
cleanup kill when the filter is removed.

Clear (zero) the reused filters (ap_filter_t) on reuse rather than cleanup,
then a single APR_RING_CONCAT() can be used to recycle dead_filters in a one
go.

Always call ap_filter_recycle() in ap_filter_output_pending(), even if no
filter is pending, and while at it fix s/ap_filter_recyle/ap_filter_recycle/
silly typo.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1840028 13f79535-47bb-0310-9956-ffa450edef68