[runtime] Implement the wait in mono_runtime_class_init_full () using a cond variable...

[runtime] Implement the wait in mono_runtime_class_init_full () using a cond variable instead of a mutex.

Previously it waited for the initialization to be done using a mutex lock/unlock, which needed some additional checks before acquiring the lock. Use a normal cond variable instead. Remove some code duplication from the lock cleanup code.