Refactor circuit_predict_and_launch_new()

The logic in circuit_predict_and_launch_new() is complex and can be refactored to be easier to understand and extend.

sysrqb took a stab at refactoring it in legacy/trac#13239 (moved), but we should probably look at the whole function from the beginning and try to improve its codeflow in general.

It's also likely that the behavior of the function can be improved and some research should be done in that area as well.