BUG/MEDIUM: dns: wrong first time DNS resolution

First DNS resolution is supposed to be triggered by first health check,
which is not the case with current code.
This patch fixes this behavior by setting the
resolution->last_resolution time to 0 instead of now_ms when parsing
server's configuration at startup.
diff --git a/src/server.c b/src/server.c
index e88302b..f3b0f16 100644
--- a/src/server.c
+++ b/src/server.c
@@ -978,7 +978,7 @@
 			curr_resolution->status = RSLV_STATUS_NONE;
 			curr_resolution->step = RSLV_STEP_NONE;
 			/* a first resolution has been done by the configuration parser */
-			curr_resolution->last_resolution = now_ms;
+			curr_resolution->last_resolution = 0;
 			newsrv->resolution = curr_resolution;
 
  skip_name_resolution: