Add support for running canvas micro tests
Allow specifying flexible_command options to the sample command
Switch from leveldb to a json file

The leveldb directory is quite tricky to maintain in the repository and
the amount of data that we are handling doesn't necessitate something so
speedy.
Do all sample processing eagerly

Instead of applying normalization and millisecond to second conversion
in the UI, apply it when parsing test results. This makes it easier to
handle micro tests, which do not have this concept.