Remove all function pointers from the ap_bucket type. These function

Remove all function pointers from the ap_bucket type.  These function
pointers are replaced with a global table that allows modules to register
their bucket types.  Those bucket types are then allowed to be used in
the server processing.  This also required removing all direct calls to
those functions.  The ap_bucket type has an index into an array, so in
each ap_bucket_* function, we use that index to find the correct set of
functions.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86575 13f79535-47bb-0310-9956-ffa450edef68