Relax promises about condition variable.
Relax promises about condition variable.

This allows for futex or thread parking based implementations in the
future.

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust