Use supervisor to manage condition variables

[jvoegele]

Currently, condition variables are implemented as an unsupervised GenServer process that is lazily initialized on first use, whether that is in the signal macro or as a :signal option in one of the wait* macros. Furthermore, the condition variable process is linked to the process that first used it and if this process exits it will also cause the condition variable process to exit even if other processes are still relying on it.

To fix this issue, condition variables should be started by and linked to a supervisor process instead of the (arbitrary) first process that happens to use the condition variable.

[jvoegele]

Resolved by Pull Request #7