Fix for proxy_store leaving temporary files for subrequests.

Fix for proxy_store leaving temporary files for subrequests.

Temporary files might not be removed if the "proxy_store" or "fastcgi_store"
directives were used for subrequests (e.g. ssi includes) and client closed
connection prematurely.

Non-active subrequests are finalized out of the control of the upstream
module when client closes a connection.  As a result, the code to remove
unfinished temporary files in ngx_http_upstream_process_request() wasn't
executed.

Fix is to move relevant code into ngx_http_upstream_finalize_request() which
is called in all cases, either directly or via the cleanup handler.