Changed default value of sendfile_max_chunk to 2m.

The "sendfile_max_chunk" directive is important to prevent worker
monopolization by fast connections.  The 2m value implies maximum 200ms
delay with 100 Mbps links, 20ms delay with 1 Gbps links, and 2ms on
10 Gbps links.  It also seems to be a good value for disks.

src/http/ngx_http_core_module.c

diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index bad43ea..fe1da45 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -3720,7 +3720,7 @@
     ngx_conf_merge_value(conf->internal, prev->internal, 0);
     ngx_conf_merge_value(conf->sendfile, prev->sendfile, 0);
     ngx_conf_merge_size_value(conf->sendfile_max_chunk,
-                              prev->sendfile_max_chunk, 0);
+                              prev->sendfile_max_chunk, 2 * 1024 * 1024);
     ngx_conf_merge_size_value(conf->subrequest_output_buffer_size,
                               prev->subrequest_output_buffer_size,
                               (size_t) ngx_pagesize);