Allow integrating third party logger

[mrbald]

For pretty much any production usage you'd want to redirect all logging into the application log file (not std::cerr).
Would be great to have a way to integrate a third party logger, can be as simple as a <severity, message> callback.

[mzimbres]

Hi,

the original plan was to allow users pass their custom loggers to basic_connection::async_run where the logger interface follows the redis::logger plublic api. This API however is unpolished and is likely to change in the future. It is also highly inconvenient to use the basic_connection instead of connection because of compilation times. At the moment however this is the only workaround.