Recovery fixes

#### Recovery
* made correct stopping and closing pool of processes and monitor threads
* use portable copy of recovery context in subprocesses and removed global context
* moved initialisation and destruction of processes pool into recovery.py - removed duplicating code
* fixed memory leak at merge recovery described at https://github.com/reverbrain/elliptics/issues/552
* speedup merge from dump recovery - use cached route-list for routing
* implemented 'one-node' merge from dump recovery

#### Tests
* decreased number of backends and nodes at pytests - decreased total number of threads and processes that will be created and used from pytests

### Python binding:
* removed wrapping address method at result entries - it is needless because python binding represents inheritance relationships between result entries and  CallbackResultEntry.