Proposal: promise FIFO (queue operation)

[lea108]

The docs of pool package doesn't make any promises on in witch order scheduled requests are processed.

To reduce risks of gridlocks it helps to know in which order the requests are processed.

I looked in the implementation and it looks like it uses a Queue and thus could make it external that requests are processed in a FIFO order. I understand it makes it harder to change later if it is public, but it is useful information to avoid grid locks in the app processing to know which order requests are processed.