MINOR: log: Provide a function to emit a log for an application

MINOR: log: Provide a function to emit a log for an application

Application is a generic term here. It is a modules which handle its own log
server list, with no dependency on a proxy. Such applications can now call the
function app_log() to log messages, passing a log server list and a tag as
parameters. Internally, the function __send_log() has been adapted accordingly.