Fix thread names in dbt server logs

[drewbanin]

Describe the bug

The dbt server logging output shows "QueueFeederThread" as the source for all log output. This isn't correct - the logs are actually being emitted by threads named things like master or Thread-3. Let's update the log handling code to print the correct thread name in the structured logs.

This change will help us better understand:

1. parallelized execution in dbt
2. per-thread connections and query pooling

Expected behavior

The thread_name in structured log output in the dbt server responses (like from poll) should render out as master or Thread-N or similar, not QueueFeederThread.

Screenshots and log output

The output of dbt --version:

0.15.0-b3

The operating system you're using:
macOS

The output of python --version:
Py3.7.5