remove QUEUE_TIMEOUT_GET, use a Stop sentinel instead

The QUEUE_TIMEOUT_GET causes some unnecessary slowdowns.  By removing it and using a sentinel instead to end the threads, several s3-based operations can run faster.

Old:
% time aws s3 cp empty_file s3://test_bucket/empty_file
upload: ./empty_file to s3://test_bucket/empty_file
aws s3 cp empty_file s3://test_bucket/empty_file 0.31s user 0.05s system 16% cpu 2.189 total

New:
% time aws s3 cp empty_file s3://test_bucket/empty_file
upload: ./empty_file to s3://test_bucket/empty_file
aws s3 cp empty_file s3://test_bucket/empty_file 0.27s user 0.06s system 85% cpu 0.385 total