Raised simultaneous subrequest limit from 50 to 200.

It wasn't enforced for a long time, and there are reports that people
use up to 100 simultaneous subrequests now.  As this is a safety limit
to prevent loops, it's raised accordingly.

src/http/ngx_http_request.h

diff --git a/src/http/ngx_http_request.h b/src/http/ngx_http_request.h
index 17c4b1e..70ca609 100644
--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -10,7 +10,7 @@
 
 
 #define NGX_HTTP_MAX_URI_CHANGES           10
-#define NGX_HTTP_MAX_SUBREQUESTS           50
+#define NGX_HTTP_MAX_SUBREQUESTS           200
 
 /* must be 2^n */
 #define NGX_HTTP_LC_HEADER_LEN             32