TBS: potential data loss in race condition between event arrival and receiving decision

[carsonip]

As TBS pebble implementation does not use db transaction and there are no locks, theoretically it is possible for the following sequence of events to happen.

1. goroutine A: a trace event T belonging to trace id 123 to arrive
2. goroutine A: confirm that no decision is made for trace id 123
3. goroutine B: make a sampling decision for trace id 123
4. goroutine B: publish all events for trace id 123
5. goroutine A: write trace event T to pebble
6. trace event T will sit in pebble until being deleted at TTL

Confirm if this is possible by adding a unit test, and if so, add a fix and measure the perf hit.