Decreased the NGX_HTTP_MAX_SUBREQUESTS limit.

There is no much sense in such a big value since its semantics
has been changed in 06e850859a26 to limit recursive subrequests.

src/http/ngx_http_request.h

diff --git a/src/http/ngx_http_request.h b/src/http/ngx_http_request.h
index 7e56c39..eac8b6d 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           200
+#define NGX_HTTP_MAX_SUBREQUESTS           50
 
 /* must be 2^n */
 #define NGX_HTTP_LC_HEADER_LEN             32