next round of changes.

cumulative commit:
1. get rid of load_balance() in favor of lb_*() family of functions to make the workflow more clear and defined:
       lb_start() - strictly used to start LB session. if sessions already started, old session will be lost and re-started again.
       lb_next() - strictly used to to continue LB session previousely started by lb_start().
       lb_is_started() - used to check whenever LB session is already started or not.
       lb_reset() - used to clean up LB session and everything LB internally does for this dialog.
       ln_disable_dst() - mark destination currently choosen by LB in current active LB session as disabled.
       lb_start_and_next() - is simply a shorthand for 'lb_is_started()?lb_next():lb_start()' to simplify scripting.
                 this function logically almost the same as old load_balance() function,
                 so we could leave old name for compatibility reasons.
       lb_count_call() - manually register call as a load for particular resource(es).

2. lb_start(..., alg) parameter changed to 'flags' parameter with possible values:
      'r' - use relative versus absolute estimation
      'n' - do not skip negative loads

3. new module parameter 'probing_verbose' to log INFO message whenever we enable/disable destinations by probing/MI.