Added tracbuildbot/tools.py with Singleton superclass.
tracbuildbot/buildbot_api.py: added tests from buildbot_api_tests.py and some fixes.

- also uses new Singleton from tools;
- added check changes of address to connect_to;
- etc.
Added tracbuildbot/buildbot_cache.py(instead of cache.py) with cache realization using trac db.

- also realized deferred cache uses multiprocessing module;
- modified tracbuildbot/timeline.py to use DeferredBuildbotCache.
tracbuildbot/buildbot_api.py: BuildbotConnection - added method reconnect.
tracbuildbot/buildbot_cache.py: rewrited async part using multiprocessing.Pool.