Simplify the directory/file sync loop

@sueloverso, if you like this change please merge, otherwise discard it.

Basically `did_sync` was bugging me, so I thought we could break out at the bottom of the loop instead?

And, there were a few places where we ignored error returns (the call to `__wt_cond_wait`, and after `__wt_directory_sync` failed, we still cleared SLOT_SYNC_DIR from the slot).

Anyway, for your consideration.