Fix MaxConnectionsPerChild

This was broken when the handling of lingering close was moved into the
listener thread.

- Make the connection counting thread safe.
- Do the counting in the connection pool cleanup to ensure that it gets
  also executed if the listener thread closes the connection.
- Add a trace log message when a process is recycled.
- Rename requests_this_child to conns_this_child, which is more accurate

Backport of r1343085 and r1343087.

Submitted by: sf
Reviewed by: humbedooh, rjung
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1364268 13f79535-47bb-0310-9956-ffa450edef68