Resolver: cancelable resend timer event.

It is safe because re-sending still works during graceful shutdown as
long as resolving takes place (and resolve tasks set their own timeouts
that are not cancelable).

Also, the new ctx->cancelable flag can be set to make resolve task's
timeout event cancelable.

src/core/ngx_resolver.c

diff --git a/src/core/ngx_resolver.c b/src/core/ngx_resolver.c
index d3747d1..cd55520 100644
--- a/src/core/ngx_resolver.c
+++ b/src/core/ngx_resolver.c
@@ -191,6 +191,7 @@
     r->event->handler = ngx_resolver_resend_handler;
     r->event->data = r;
     r->event->log = &cf->cycle->new_log;
+    r->event->cancelable = 1;
     r->ident = -1;
 
     r->resend_timeout = 5;
@@ -4008,6 +4009,7 @@
     ctx->event->handler = ngx_resolver_timeout_handler;
     ctx->event->data = ctx;
     ctx->event->log = r->log;
+    ctx->event->cancelable = ctx->cancelable;
     ctx->ident = -1;
 
     ngx_add_timer(ctx->event, ctx->timeout);

src/core/ngx_resolver.h

diff --git a/src/core/ngx_resolver.h b/src/core/ngx_resolver.h
index 6f099b7..0bd3921 100644
--- a/src/core/ngx_resolver.h
+++ b/src/core/ngx_resolver.h
@@ -220,6 +220,7 @@
 
     unsigned                  quick:1;
     unsigned                  async:1;
+    unsigned                  cancelable:1;
     ngx_uint_t                recursion;
     ngx_event_t              *event;
 };