Use a mutex to control read/write for IVM values, so we can reuse the existing...

Use a mutex to control read/write for IVM values, so we can reuse the existing structures without running into race conditions.
This should get rid of the need to have MaxConnectionsPerChild set to > 0.

If a new value is set and is a string, we either use the existing varbuf or grow it accordingly.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1470156 13f79535-47bb-0310-9956-ffa450edef68