lightningd/plugin_hook.c: Make db_write a chained hook.

Fixes: #4219

Changelog-Changed: Plugins: Multiple plugins can now register `db_write` hooks.

tests/plugins/dbdummy.py

@@ -0,0 +1,16 @@
+#! /usr/bin/env python3
+'''This plugin is a do-nothing backup plugin which just checks that we
+can handle multiple backup plugins.
+'''
+
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook('db_write')
+def db_write(plugin, **kwargs):
+    return {'result': 'continue'}
+
+
+plugin.run()