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/tests/test_1_online.py

@@ -44,21 +44,21 @@ def test_configure_generate_key(acfactory, lp):
     lp.sec("ac1: send unencrypted message to ac2")
     chat.send_text("message1")
     lp.sec("ac2: waiting for message from ac1")
-    msg_in = ac2._evtracker.wait_next_incoming_message()
+    msg_in = ac2.wait_next_incoming_message()
     assert msg_in.text == "message1"
     assert not msg_in.is_encrypted()
 
     lp.sec("ac2: send encrypted message to ac1")
     msg_in.chat.send_text("message2")
     lp.sec("ac1: waiting for message from ac2")
-    msg2_in = ac1._evtracker.wait_next_incoming_message()
+    msg2_in = ac1.wait_next_incoming_message()
     assert msg2_in.text == "message2"
     assert msg2_in.is_encrypted()
 
     lp.sec("ac1: send encrypted message to ac2")
     msg2_in.chat.send_text("message3")
     lp.sec("ac2: waiting for message from ac1")
-    msg3_in = ac2._evtracker.wait_next_incoming_message()
+    msg3_in = ac2.wait_next_incoming_message()
     assert msg3_in.text == "message3"
     assert msg3_in.is_encrypted()