LTM support running on f1-micro instance

[tytso]

The memory consumption of the LTM currently is too high to run on an f1-micro instance.

To reduce the memory consumption, the LTM may need to use threads instead of processes for shards. The TestRunManager can still run in a separate process. For this to work, shard threads should be changed to not modify any process-level values (stdout/stderr, and root logger).
This will require changing the logging setup within a shard, as they currently replace the handlers on the root logger instead of using their own logger instance, and redirect stdout/stderr to the logfile.

Additional consideration may be necessary to coordinate access to shared variables by shard threads.

Current memory consumption approximations.
Test run with 3 shards: spikes to 200MB, stabilizes at ~100MB
Test run with 13 shards: spikes to 600MB, stabilizes at ~300MB
Approximate memory consumption per shard:
Initial spike ~50MB
Stable ~20MB
One test run process consumes ~50MB on its own.