API change: make the entropy buffers easier to use.

This commit makes the old Accumulator.AddRandomEvent() method private
and provides two new methods .NewEntropyDataSink() and
.NewEntropyTimeStampSink(), instead.  These two methods automate the
allocation of entropy source numbers and the distribution of entropy
over the different pools.

Other changes:

- Allow reseeding after 32 bytes are written to pool 0 (was 48 bytes).
  If limited entropy is available, this change reduces the risk that
  the environmental entropy is never used, because the program is
  terminated/restarted before the first reseeding.

- Add some trace messages with path fortuna/entropy to allow
  monitoring collection of entropy.

- Documentation fixes