Create Sqlite Exporter (meta-pytorch#1930)

Summary:

Stack context:
```
Our tracing subscriber has 3 layers:
- File logging
- Scuba
- Sqlite (usually off)

Although the actual Scuba logging is done in a background thread and we are using a non-blocking file writer, we still have a good chunk of work that happens for events & spans. The solution to this, is to create a `UnifiedLayer` that just sends everything into a background worker, that then delivers all traces to each `Exporter` to handle.

In this diff, we will create an initial `UnifiedLayer` and incrementally move each existing layer into an `Exporter`.

To test correctness, we will run both the old and unified implementations for initializing telemetry on a variety of workloads, and ensure that both are producing the same results
```

In this diff, we will implement an `Exporter` meant to replace `get_reloadable_sqlite_layer()`. We will update our correctness test to query the tables at the end of each workload and ensure all rows are the same

Differential Revision: D87363774

Author: thomasywang