Extended python binding, changed recovery and fixed reading ext header of object.

  * Added set_keepalive method to elliptics.Node, merge_indexes and recover_index to elliptics.Session
  * Replaced merging index shards at dc_recovery by calling elliptics.Session.merge_indexes
  * Added checking size of object before reading ext header. There was issues without this check if user try to read object that was prepared but hasn't been committed, dnet_ioserv hangs on sending "READ" replies.
  * Fixed test_session_indexes.