This change adds "http_429" parameter to "proxy_next_upstream" for
retrying rate-limited requests, and to "proxy_cache_use_stale" for
serving stale cached responses after being rate-limited.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

This change adds reason phrase in status line and pretty response body
when "429" status code is used in "return", "limit_conn_status" and/or
"limit_req_status" directives.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>

When a slice subrequest was redirected to a new location, its context was lost.
After its completion, a new slice subrequest for the same slice was created.
This could lead to infinite loop.  Now the slice module makes sure each slice
subrequest starts output with the slice context available.

Previously, if slice main request write handler was called while a slice
subrequest was running, a new subrequest for the same slice was started.

With post_action or subrequests, it is possible that the timer set for
wev->delayed will expire while the active subrequest write event handler
is not ready to handle this.  This results in request hangs as observed
with limit_rate / sendfile_max_chunk and post_action (ticket #776) or
subrequests (ticket #1228).

Moving the handling to the connection event handler fixes the hangs observed,
and also slightly simplifies the code.