Reactor-based threading model

* Use Reactor+Thread (common/thread.h) to replace existing
  libchrome-based message_loop_thread
* Use Handler to implement multiple message queue per thread, by using
  kernel-based eventfd

Test: run unit test, and run benchmark
Change-Id: Idd2e4ef99fb9a7b2c0956de0e372c67a1098f1b6