Fix so the generated txids are lexically sortable over time.

The first few bytes of the txid didn't contain the data for the entire
timestamp, so it was causing the generated txid to not be lexically
sortable as intended.

Add tests to try and better ensure that the txids are in fact sortable
and increment over time as expected.
Merge pull request #3 from GUI/sortable-fix

Fix so the generated txids are lexically sortable over time