feature: add exit_worker as new phase

Author: rainingmaster

lib/resty/core/phase.lua

@@ -28,6 +28,7 @@ local context_names = {
     [0x0400] = "ssl_cert",
     [0x0800] = "ssl_session_store",
     [0x1000] = "ssl_session_fetch",
+    [0x2000] = "exit_worker",
 }