Remove RPC based server test in C++: fixes #322

Moved test from servers to core/driver, and
refactored test and related code. In addtion,
deleted config files for servers.
This commit has some fixes, double-free bug in stat,
and ensure to clear weight_manager in driver.
Fix stat to serialize field: fixes #320
catchup develop
Merge branch 'feature/nearest_neighbor_and_push_mixer' of http://github.com/jubatus/jubatus into feature/nearest_neighbor_and_push_mixer