datastore: plugin to replicate datastore commands before next release.

See https://github.com/ElementsProject/lightning/pull/4674
(I manually tested that it passes all the tests there, too!)

When they finally upgrade the node, this automatically puts any
datastore data into the inbuilt datastore and shuts down.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

datastore/datastore.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin, RpcError
+import shelve
+import os
+
+
+plugin = Plugin()
+
+
+def unload_store(plugin):
+    """When we have a real store, we transfer our contents into it"""
+    try:
+        datastore = shelve.open('datastore.dat', 'r')
+    except:
+        return
+
+    plugin.log("Emptying store into main store!", level='unusual')
+    for k, d in datastore.items():
+        try:
+            plugin.rpc.datastore(k, d.hex())
+        except RpcError as e:
+            plugin.log("Failed to put {} into store: {}".format(k, e),
+                       level='broken')
+    datastore.close()
+    plugin.log("Erasing our store", level='unusual')
+    os.unlink('datastore.dat')
+
+
+@plugin.init()
+def init(options, configuration, plugin):
+    # If we have real datastore commands, don't load plugin.
+    try:
+        plugin.rpc.help('datastore')
+        unload_store(plugin)
+        return {'disable': 'there is a real datastore command'}
+    except RpcError:
+        pass
+
+    # Start up real plugin now
+    plugin.rpc.plugin_start(os.path.join(os.path.dirname(__file__),
+                                         "datastore-plugin.py"))
+    return {'disable': 'no builtin-datastore: plugin loaded'}
+
+
+plugin.run()