merge r3137, r3198, r3199, r3353, r3370, r3371, r3398, r3399:

merge r3137, r3198, r3199, r3353, r3370, r3371, r3398, r3399:

cache related fixes:

*) do not pass buf with empty cached response,
   this fixes "zero size buf in output" alert
*) hide cacheable Set-Cookie and P3P FastCGI response headers
*) test comma separator in "Cache-Control"
*) a cache manager thread handle was overwritten by a cache loader thread
   handle, this caused an exit delay, the bug had been introduced in r3248
*) fix handling cached HTTP/0.9 response
*) log proxied HTTP/0.9 responses status as "009"
*) fix the "If-None-Match" header name
*) fix a cached zero-length body case