MINOR: checks/event_hdl: SERVER_CHECK event

Adding a new event type: SERVER_CHECK.

This event is published when a server's check state ought to be reported.
(check status change or check result)

SERVER_CHECK event is provided as a server event with additional data
carrying relevant check's context such as check's result and health.
diff --git a/src/event_hdl.c b/src/event_hdl.c
index 5fc1935..7641486 100644
--- a/src/event_hdl.c
+++ b/src/event_hdl.c
@@ -31,6 +31,7 @@
 	{"SERVER_DOWN",         EVENT_HDL_SUB_SERVER_DOWN},
 	{"SERVER_STATE",        EVENT_HDL_SUB_SERVER_STATE},
 	{"SERVER_ADMIN",        EVENT_HDL_SUB_SERVER_ADMIN},
+	{"SERVER_CHECK",        EVENT_HDL_SUB_SERVER_CHECK},
 };
 
 /* internal types (only used in this file) */