Keep track of SQ entries, in a 1:1 relationship

[JackKelly]

Instead of keeping track of things like GetRanges in flight, maybe keep track of our own representation of SQEs.

Pros: Might simplify our code? e.g.

• we won't need to split the uring user_data into an index and an opcode, because the index is all we need (the index will take us to an object which only represents a single SQE). So we could get rid of our UringUserData struct.
• Our Operation code won't need a conditional for different opcodes.
• We could, again, just keep track of things by passing a raw pointer through the io_uring user_data. So we could get rid of Tracker. But need to benchmark.

Need to think how this will work when we optimise byte ranges. Maybe optimise byte ranges before implementing this?