Add an allocator-passing mechanism throughout the bucket brigades API.

From Apache's standpoint, the apr_bucket_alloc_t* used throughout a given
connection is stored in the conn_rec by the create_connection hook.  That
means it's the MPM's job to optimize recycling of apr_bucket_alloc_t's --
the MPM must ensure that no two threads can ever use the same one at the
same time, for instance.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94304 13f79535-47bb-0310-9956-ffa450edef68