dillon1000/react

Use EventPriority to track update priority (#21082)

Instead of LanePriority. Internally, EventPriority is just a lane, so
this skips an extra conversion. Since EventPriority is a "public" (to
the host config) type, I was also able to remove some deep imports
of the Lane module.

This gets us most of the way to deleting the LanePriority entirely.