Make StatsLog drop less.

+ Create a thread-safe LogEventQueue to buffer log events.

+ The socket listner thread will read from socket and write to the buffer as quickly as possible
  to minimize the data loss in socket.

+ All pushed data is fetched from the the buffer and processed in a dedicated thread. After an
  event is fetched from the queue, we no longer block the socket listener thread.

+ Report event queue stats via statsdstats, including the min and max queue event history span in
  the queue (to understand how slow statsd can be and how fast the events can be)

Bug: 119031518
Test: unit tests added in statsd_test

Change-Id: I6b65ed9a678935b2e24302ba4b36e69c157adde4