This commit introduces two changes. First, it changes type of experiment
id from UUID to timestamp, mainly to reuse existing id and improve
readability of events. This simplifies experiment analysis when the db
contains results from many tests.

Second, it adds initital test framework to validate event stream
generate by benchmark executor. The test does not really execute any sql
statements. A mock simulates the execution so that just the executor's
behavior can be validated.

Addresses microsoft#53