tests: reduce number of io/net threads
recovery: decreased log level of log that looking up key from dump fail failed.
recovery: used threading.Event for waiting complete.
Merge pull request #551 from shaitan/master

Recovery
Merge branch 'master' of github.com:reverbrain/elliptics
cmake: added blackhole headers lookup
test: decreased number of backends and nodes at pytests - decreased number of threads and processes used from tests.
recovery: correctly stops monitor threads.
recovery: correctly close and join process pool.
recovery: cleanups global reference to context.
recovery: implemented one-node recovery for merge from dump.
Recovery: Use cached route-list for speedup recovering.
recovery: Moved code from dnet_recovery to recovery.py for using/testing it from pytests.
Added 'portable' method to Ctx that returns lightweight copy of the context that can be used by multiprocessing.
Removed monitor from context, left only stats - proxy object for sending updates to monitor from different processes/threads.
Removed using global context, use portable context insead of.
Moved process pool initialization and destruction into common recovery.py - removed duplicating code.
python: removed wrapping address method at result entries that inherited from CallbackResultEntry.
recovery: fixed memory leak at merge recovery.
recovery: removed odd increasing of iterations.
Merge pull request #554 from shaitan/master

Recovery fixes
v2.26.3.30
Fixed remove_on_fail implementation

We need to clone session to be sure that the session will be used only
in one transaction simultaneously
addr: new thread-safe helpers to print address strings