Improvements to the Accumulator.Close() method.

This commit implements two separate improvements to the shutdown
process of Accumulators:

1) Before writing the new seed file, the .Close() method now transfers
  all entropy remaining in the entropy pools into the underlying
  generator and then deallocates the pools.  This change avoids
  entropy from partially filled pools being lost on shutdown.

2) All goroutines started by NewEntropyDataSink() and
  NewEntropyTimeStampSink() are now stopped by the Accumulator.Close()
  method.  This change prevents memory leaks and also avoids race
  conditions between simultaneous deallocation of entropy pools and
  submission of new entropy.