Sending messages end to end (#21)

* duplex connection end-to-end (working, debug logs)

* agent: send, receive, acknowledge messages

* logging proposal

* logging: client/server (dis)connections

* agent scenario testing framework

* add tests, remove logs

* clean up

docs/logging.md

@@ -0,0 +1,70 @@
+# SMP agent logging
+
+## Problem and proposed solution.
+
+SMP agent performs multiple actions in response to the client commands and to the messages received from other SMP agents (wrapped in SMP protocol messages).
+
+Customary approach for the network protocol clients is to have a debug/verbose mode that enables logging of all sent and received messages and any other actions that the client performs.
+
+This document proposes a logging format for SMP agent that would be enabled if the agent is run with `--verbose` / `-v` command line option.
+
+We can also consider logging the database operations that change the data.
+
+## Types of actions and the associated log line format.
+
+### Client connected / disconnected
+
+```
+client n connected to Agent
+client n disconnected from Agent
+```
+
+where `n` is a sequential number of a connected agent client, starting from 1 (over the agent run-time).
+
+### Server connected / disconnected
+
+```
+Agent connected to host:port
+Agent disconnected from host:port
+```
+
+### Received command from the client
+
+```
+n --> A : corrId connAlias parsed_command // raw_command
+```
+
+`raw_command` is added only in case of parsing failure.
+
+### Sent command to SMP server
+
+```
+A --> host:port : corrId queueId parsed_command
+```
+
+### Received response / message from the SMP server
+
+```
+A <-- host:port : corrId queueId parsed_command // raw_command
+```
+
+In case the response is a message or notification, corrId should be replaced with `_`
+
+### Interpreted ("unwrapped") SMP message as agent message
+
+```
+Agent msg : connAlias parsed_message // raw_message 
+```
+
+### Sent response / message to the client
+
+```
+n <-- A : corrId connAlias parsed_command // raw_command
+```
+
+### Database changes
+
+```
+DB : insert/delete/update table key
+DB : insert/delete/update table key
+```