Move fileid map in SMF_Writer to SmallFileFD

So that only the file id mapping entries for open files are cached
in memory.
Merge pull request #230 from zhang-jingwang/2.4dev

Move fileid map in SMF_Writer to SmallFileFD
Merge branch '2.4dev' of github.com:plfs/plfs-core into 2.4dev
Moved PLFSIndex to LogicalFS