tests: reduce number of io/net threads
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.