New APIs for message processing loops

This patch adds new C APIs
dc_get_next_msgs() and dc_wait_next_msgs(),
and their JSON-RPC counterparts
get_next_msgs() and wait_next_msgs().

New configuration "last_msg_id"
tracks the last message ID processed by the bot.
get_next_msgs() returns message IDs above
the "last_msg_id".
wait_next_msgs() waits for new message notification
and calls get_next_msgs().
wait_next_msgs() can be used to build
a separate message processing loop
independent of the event loop.

Async Python API get_fresh_messages_in_arrival_order()
is deprecated in favor of get_next_messages().

Introduced Python APIs:
- Account.wait_next_incoming_message()
- Message.is_from_self()
- Message.is_from_device()

Introduced Rust APIs:
- Context.set_config_u32()
- Context.get_config_u32()

python/src/deltachat/message.py

@@ -344,6 +344,16 @@ class Message:
         contact_id = lib.dc_msg_get_from_id(self._dc_msg)
         return Contact(self.account, contact_id)
 
+    def is_from_self(self):
+        """Return true if the message is sent by self."""
+        contact_id = lib.dc_msg_get_from_id(self._dc_msg)
+        return contact_id == const.DC_CONTACT_ID_SELF
+
+    def is_from_device(self):
+        """Return true if the message is sent by the device."""
+        contact_id = lib.dc_msg_get_from_id(self._dc_msg)
+        return contact_id == const.DC_CONTACT_ID_DEVICE
+
     #
     # Message State query methods
     #