CopyOnWrite directory should clear working set of files being written upon start

CopyOnWriteDirectory makes use of working set (introduced with OAK-3110) to notify the CopyOnReadDirectory that those files are in use. This would ensure that CoR dir does not consider such files as garbage.

This set gets cleared when CoW dir closes cleanly. However in case it does not close cleanly such a state would not be cleared. To handle such cases we should clear the working set upon construction of CoW dir as there can be only one writer by design