MPMs unix: bind the bucket number of each child to its slot number

We need not remember each child's bucket number in SHM for restarts, for the
lifetime of the httpd main process the bucket number can be bound to the slot
number such that: bucket = slot % num_buckets.

This both simplifies the logic and helps children maintenance per bucket in
threaded MPMs, where previously perform_idle_server_maintenance() could create
or kill children processes for the buckets it was not in charge of.

Submitted by: ylavic
Reviewed by: ylavic, rpluem, jorton


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1855378 13f79535-47bb-0310-9956-ffa450edef68