Separate cpuworker queues by handshake type

Now we prioritize ntor create cells over tap create cells.

Starts to address ticket 9574.
refactor and give it unit tests
add info-level logs to help track onion queue sizes
check bounds on handshake_type more thoroughly
do a lopsided round-robin between the onion queues

that way tap won't starve entirely, but we'll still handle ntor requests
quicker.
let the NumNTorsPerTAP consensus param override our queue choice
Be more general in calculating expected onion queue processing time

Now we consider the TAP cells we'll process while draining the NTor
queue, and vice versa.
add a changes entry for ticket 9574
nickm wants us to prioritize tap in a currently-rare edge case