mpm_worker: follow up to r1783755.

Use a mutex for ptrans' allocator to be safe with concurrent
creation and destruction of its subpools, like with mod_http2.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1783808 13f79535-47bb-0310-9956-ffa450edef68