Add generic support for reference-counting the resources used by

buckets, and alter the HEAP and MMAP buckets to use it. Change
the way buckets are initialised to support changing the type of
buckets in place, and use it when setting aside TRANSIENT buckets.
Change the implementation of TRANSIENT buckets so that it can be
mostly shared with IMMORTAL buckets, which are now implemented.

Reviewed by:	rbb


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86115 13f79535-47bb-0310-9956-ffa450edef68