Goroutines leak in WithRootCancel

[richhx]

Whenever a child context is created via WithRootCancel, a goroutine
spawns to gracefully handle closing the child context whenever the root
context gets canceled.

However, the current implementation leaks goroutines whenever the child
context exits before the root context exits.

[richhx]

See #214 for the test reproduction. Happy to provide pprof metrics as needed too.