Provide ConditionVariableTimedSleep(), like ConditionVariableSleep()
but with a timeout argument.

Author: Shawn Debnath
Reviewed-by: Kyotaro Horiguchi, Thomas Munro
Discussion: https://postgr.es/m/eeb06007ccfe46e399df6af18bfcd15a@EX13D05UWC002.ant.amazon.com