Replace logging backend to log4rs and add log rotation (#1789)

* Replace logging backend to flexi-logger and add log rotation
* Changed flexi_logger to log4rs
* Disable logging level filtering in Root logger
* Support different logging levels for file and stdout
* Don't log messages from modules other than Grin-related
* Fix formatting
* Place backed up compressed log copies into log file directory
* Increase default log file size to 16 MiB
* Add comment to config file on log_max_size option

api/src/lib.rs

@@ -33,7 +33,7 @@ extern crate serde;
 extern crate serde_derive;
 extern crate serde_json;
 #[macro_use]
-extern crate slog;
+extern crate log;
 extern crate futures;
 extern crate http;
 extern crate hyper_rustls;