improved for rpc requests logging, several minor improvements in rpc_server.cpp

Improved rpc requests logging, now it outputs more data (so it's easier to debug rpc calls) and uses a separate logger named "rpc". In a calling app it can be configured to dump data either to a file or to stdout; or if not configured at all it will just skip these logging at all.
Also I made rpc_server::config::is_valid() to return true even if username and password fields are empty - this way I can skip http basic auth when using it from QT app.
Among other changes now it skips everything after '?' sign in filenames, so for example /index.html?version=123 would be a valid file name - sometimes front ends add some stuff after ? to make browsers to reload not cached version of a file.
_I did some basic tests, but apparently more testing needed_