Storage Node Watchdog needs to check journal directory

Since the journal directory is often moved to other storage devices, separate from the db files, it is important for the storage node watchdog to check this directory.

In the mmapv1 case, the journal is computed here:
https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/mmap_v1/dur_journal.cpp#L111-L115

In the WT case, the journal is computed here:
https://github.com/mongodb/mongo/blob/master/src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine.cpp#L275