Implement [ap_]atomics for [u]int/[u]long/size_t and use them for (some) proxy_worker_shared fields

[ylavic]

Follow up to https://lists.apache.org/thread/np1xzvq6yq5onmtqqovyx0doncowvp3f

This is a proposal/POC to implement [ap_]atomics on system dependent signed/unsigned types, using the uint32/uint66/ptr atomics from the APR as primitives.

This currently implements:

• ap_atomic_{int,long,size}_get()
• ap_atomic_{int,long,size}_set()
• ap_atomic_{int,long,size}_xchg()
• ap_atomic_{int,long,size}_cas()
• ap_atomic_{int,long,size}_add()
• ap_atomic_{int,long,size}_add_sat()
• ap_atomic_{int,long,size}_sub()
• ap_atomic_{int,long,size}_sub_sat()

The add/sub functions wrap on under/overflow (safely for signed types), while the add_sat/sub_sat ones saturate instead (e.g. at INT_MIN/INT_MAX for ints, at 0/ULONG_MAX for unsigned longs, ...).

This allows to replace/remove the ap_proxy_{get,set,dec,inc}_busy_count() specialized functions by using the new ap_atomic_size_*() helpers directly on worker->s->busy, and likewise to use the the new ap_atomic_int_*() directly on worker->s->lbstatus to switch to atomic ops there too.

TODO: dox, MMNs..

[jfclere]

Oops I caused the conflicts.
Note I have created #421 that works around a problem when building with windows.
I think that is due to the changes in httpd.h.