mpm_event(opt), mpm_worker, mpm_prefork: follow up to r1635521, r1640161.

mpm_event(opt), mpm_worker, mpm_prefork: follow up to r1635521, r1640161.

Retain num_buckets and max_buckets accross restarts so that we can determine
whether new buckets were allocated and set their idle_spawn_rate at the same
level as the existing ones (max).

Also, adjust ap_daemons_limit and ap_daemons_to_start lower bounds at mpm_run()
time, once num_buckets is available for the current generation (previously done
at check_config time, hence before num_buckets is computed, and then with the
previous generation's value).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1640763 13f79535-47bb-0310-9956-ffa450edef68