Add keepalive messages for long running commands to prevent client timeouts

[benmcclelland]

Describe the solution you'd like Some commands like complete multipart upload can take a long time on some filesystems and possibly timeout on the client side. This causes the clients to have to set long timeouts for these commands to be successful.

We need to investigate if there are messages that can be sent back to the client periodically to let it know that the server is still processing the command.