HTTP connections less persistent with >1 worker

[docbert]

s5cmd supports HTTP persistent connections, which are good for performance (especially for HTTPS), however when using >1 workers the number of requests sent per connection drops significantly, and thus the number of new TCP connections opened goes up. This add latency and overhead due to the additional TCP connections, but also due to TLS negotiation that needs to occur on each new connection.

Testing using s5cmd cp to upload 50,000 objects to an S3 bucket with --workers 1 results in a 2 TCP connection being opened, and half of the requests being sent over each connection.

The same test with --workers 16 results in 10,500 TCP connections being opened, and an average of ~5 requests per connection (although they vary from 1 to around 10 requests per connection). With 64 workers it goes up to ~15,000 connections.

tcpdump confirms the connections are being closed by the client-end, and they are being closed immediately after the previous successful PUT request.