say: introduce on_log_level routines

Let's introduce on_log_level static variable which is assumed to be
configured in `say_set_log_callback()`. on_log_level is assumed to be
log level of `log->on_log` callback (i.e. if entry to be logger features
higher log level - it is simply skipped). Note that now casual log_level
is calculated as MAX(level, on_log_level) since log_level is the single
guard for passing execution flow to `log_vsay()` where both things (to
be precise on_log callback invocation and ordinary logging) happens.

This change is required since if log_level has lower magnitude than
on_log_level - on_log callback will be skipped.

NO_DOC=<Internal change>
NO_TEST=<Internal change>
NO_CHANGELOG=<Internal change>