Merge r1779573 from trunk:

mod_proxy_hcheck: thread-safety.
Use the thread pool everywhere for the needs/lifetime of the request to
the backend.

Submitted by: ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1788510 13f79535-47bb-0310-9956-ffa450edef68