Recovery&monitoring

Recovery:
  * Enabled `-r` for multiple use and set `-o` to be followed by node address
  * Increased informativity of error logs with exception (used repr for exception and added traceback)
  * Added support `-f` in merge (recovering keys from dump file)
  * Updated documentation
  * Fixed splitting ranges in small pieces for merging them in different processes
  * Added saving iterated keys to dump file(s) that could be used for resuming recovery after some issues.

Core:
  * fixed hanging up iteration on cond_wait

Monitor:
  * Added clearing m_cmd_info_current after swapping

Python binding:
  * use one argument for id and context at elliptics.Session.exec_
  * made src_key as the last optional argument of elliptics.Session.exec_