Embedded perl does not set any request fields needed for conditional
requests processing.  Further, filter finalization in the not_modified
filter can cause segmentation faults due to cleared ctx as in
ticket #1786.

Before 5fb1e57c758a (1.7.3) the not_modified filter was implicitly disabled
for perl responses, as r->headers_out.last_modified_time was -1.  This
change restores this behaviour by using the explicit r->disable_not_modified
flag.

Note that this patch doesn't try to address perl module robustness against
filter finalization and other errors returned from filter chains.  It should
be eventually reworked to handle errors instead of ignoring them.

A new directive limit_req_dry_run allows enabling the dry run mode.  In this
mode requests are neither rejected nor delayed, but reject/delay status is
logged as usual.

In case of filter finalization, essential request fields like r->uri,
r->args etc could be changed, which affected the cache update subrequest.
Also, after filter finalization r->cache could be set to NULL, leading to
null pointer dereference in ngx_http_upstream_cache_background_update().
The fix is to create background cache update subrequest before sending the
cached response.

Since initial introduction in 1aeaae6e9446 (1.11.10) background cache update
subrequest was created after sending the cached response because otherwise it
blocked the parent request output.  In 9552758a786e (1.13.1) background
subrequests were introduced to eliminate the delay before sending the final
part of the cached response.  This also made it possible to create the
background cache update subrequest before sending the response.

Note that creating the subrequest earlier does not change the fact that in case
of filter finalization the background cache update subrequest will likely not
have enough time to successfully update the cache entry.  Filter finalization
leads to the main request termination as soon the current iteration of request
processing is complete.