Crash safety

[divergentdave]

I ran a stripped down version of my ALICE test harness against a recent alpha of sled, and identified some durability issues. Half of the reported items were low hanging fruit in marble's creation of a new object store. (These could also mask more subtle issues after startup) See also spacejam/sled#1229.

• The base directory should be fsynced (if on Linux) after creating the slabs and metadata subdirectories.
• The metadata directory should be fsynced after creating a new log file. It is already fsynced immediately before this, and following installation of a new snapshot via a rename.
• The slabs directory should be fsynced after creating all slab files.
• It appears that slab files are not fdatasync'd after appending to them, and this breaks the durability of sled's flush.