make it configurable whether we reset timestamps and ids in fixtures

[axiomofjoy]

• make it configurable whether we reset span ids and timestamps on trace fixtures. needed because having stable timestamps is useful for testing against features. something like a --reset-ids-and-timestamps command line argument to the trace-fixture command would be helpful.
• ensure that spans from trace fixtures are inserted into the db in (roughly) chronological rather than reverse chronological order. needed because otherwise, the fixture winds up having rowids that decrease over time, which is unrealistic and makes it difficult to test features such as pagination.

for reference, this is how i am currently getting around this issue in the pagination pr: 389bc5b#diff-bedd740bede03d53ccfd0df1d998bff39a53e43ea041a9fac2911beb90d5682b