Figure out the optimal number of sockets to use per AFD handle

[notgull]

This comment says that an arbitrary number of sockets can be used per AFD handle. However, wepoll limits it to 32, as apparently it gets slower to cancel requests the more sockets are associated with the handle. I just did 32 sockets because that's what wepoll did, but it would be nice to do some benchmarking and figure out the optimal number for our use case.