Threaded transfer hangs

[bollig]

Data transfers hang when num_transfer_threads is > 2.

Haven't started debugging yet, but it seems as though the transfer pool stalls while waiting for parts to be become available. Whatever race condition happens is avoided when 1 or 2 threads are used.

This works:

transfer:
local_temp_dir: /tmp/ngsdp
compress_transfers: False
transfer_chunk_size: 128
num_compress_threads: 4
num_transfer_threads: 1
num_decompress_threads: 4