Cache locks initial implementation.

New directives: proxy_cache_lock on/off, proxy_cache_lock_timeout.  With
proxy_cache_lock set to on, only one request will be allowed to go to
upstream for a particular cache item.  Others will wait for a response
to appear in cache (or cache lock released) up to proxy_cache_lock_timeout.

Waiting requests will recheck if they have cached response ready (or are
allowed to run) every 500ms.

Note: we intentionally don't intercept NGX_DECLINED possibly returned by
ngx_http_file_cache_read().  This needs more work (possibly safe, but needs
further investigation).  Anyway, it's exceptional situation.

Note: probably there should be a way to disable caching of responses
if there is already one request fetching resource to cache (without waiting
at all).  Two possible ways include another cache lock option ("no_cache")
or using proxy_no_cache with some supplied variable.

Note: probably there should be a way to lock updating requests as well.  For
now "proxy_cache_use_stale updating" is available.

src/http/modules/ngx_http_proxy_module.c

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index a525cd9..b65093b 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -402,6 +402,20 @@
       offsetof(ngx_http_proxy_loc_conf_t, upstream.cache_methods),
       &ngx_http_upstream_cache_method_mask },
 
+    { ngx_string("proxy_cache_lock"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_FLAG,
+      ngx_conf_set_flag_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_proxy_loc_conf_t, upstream.cache_lock),
+      NULL },
+
+    { ngx_string("proxy_cache_lock_timeout"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_msec_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_proxy_loc_conf_t, upstream.cache_lock_timeout),
+      NULL },
+
 #endif
 
     { ngx_string("proxy_temp_path"),
@@ -2435,6 +2449,8 @@
     conf->upstream.cache_bypass = NGX_CONF_UNSET_PTR;
     conf->upstream.no_cache = NGX_CONF_UNSET_PTR;
     conf->upstream.cache_valid = NGX_CONF_UNSET_PTR;
+    conf->upstream.cache_lock = NGX_CONF_UNSET;
+    conf->upstream.cache_lock_timeout = NGX_CONF_UNSET_MSEC;
 #endif
 
     conf->upstream.hide_headers = NGX_CONF_UNSET_PTR;
@@ -2681,6 +2697,12 @@
         conf->cache_key = prev->cache_key;
     }
 
+    ngx_conf_merge_value(conf->upstream.cache_lock,
+                              prev->upstream.cache_lock, 0);
+
+    ngx_conf_merge_msec_value(conf->upstream.cache_lock_timeout,
+                              prev->upstream.cache_lock_timeout, 5000);
+
 #endif
 
     if (conf->method.len == 0) {