API logging

[abitmore]

We need a way to log API calls for analysis, trouble shooting and optimization.
https://en.wikipedia.org/wiki/Server_log
https://www.sumologic.com/blog/it-operations/apache-logs-vs-nginx-logs/

[abitmore]

It turns out that rpc logging can be enabled by appending these to config.ini:

# declare an appender named "rpc" that writes messages to rpc.log
[log.file_appender.rpc]
filename=logs/rpc/rpc.log
# filename can be absolute or relative to this config file

# route messages sent to the "rpc" logger to the rpc appender declared above
[logger.rpc]
level=debug
appenders=rpc

About log level:

• level=error : logs errors
• level=warn : logs requests
• level=info : logs responses

[abitmore]

Done with #919.