core: Write Completion (WC) bucket type.

core: Write Completion (WC) bucket type.

A WC bucket is meant to prevent buffering/coalescing filters from retaining
data, but unlike a FLUSH bucket it won't cause the core output filter to
block trying to flush anything before.

It can be passed by async handlers which want to never block, followed by
ap_filter_should_yield() to check for pending data and eventually suspend
processing until MPM/asynchronous write completion finishes.

In this commit it's used that way by the tunneling loop of mod_proxy to
prevent SSL coaslescing.

gh: closes #200

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1891148 13f79535-47bb-0310-9956-ffa450edef68