Introduce the new struct inodeParents that wraps a map
and one special slot for the most recent parent. Unit tests
included.

Because the map is lazily initialized, we should save some
memory on the common single-parent case (= file with no hard links)
compared to before.

Benchmarking with gocryptfs shows no discernible change
in performance. fsstress testing with gocryptfs shows no issues.

TestStaleHardlinks the previous commit passes now.

Change-Id: I8d69093abc906addde751a9e70dbd78a3a61371a