core: tune performance (#73)

- Avoid using recursive function in critical path.
- Do not write through when using levelDB. Things put to
   levelDB would be safe from panic even we didn't force
   to write through every time.
- Dump count of confirmed blocks proposed by self.
- Avoid allocating variables in loop.
- Return length of acking node set, we only need that
   when total ordering.
- Fix potential bug: make sure win records updated when
   acking height vectors of candidates are changed.
- Keep dirty validators in slice.
- Add cache for objects to ease the pressure to garbage
   collector.
- Cache global acking status when total ordering.
- Add method to recycle blocks.
- Marshal JSON should be called once for each broadcast.
- Make updateWinRecord called in parallel.
- Log average / deviation of latencies when simulation
   finished.

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