Add a function to check that each rain stripe can successfully
reconstruct the original file. It checks every possible stripe combination to see
if the resulting file has the correct checksum.
A standalone executable is also available to manually check for theses
errors.

The fsck error map had to be changed from a map<fsid, fid> to a map<fid,
fsid> as there can be multiple invalid stripes, which all need to be
fixed at once.

Update CommitHelper to support multiple dropfsids.

Remove unused statistics collection on the fsts.

Fix buffer overflow in FsckEntry::RepairFstXsSzDiff when diskchecksum is
empty.

Fix issue with rain reconstruction where the reconstructed stripes were
invalid on non xfs filesystems.
This was caused by a bookingsize of 1024 which triggered an fallocate
before recovering the first group of the stripe. fallocate on non xfs
filesystems is done with posix_fallocate which reserves the space by
filling the file with zeroes. This then...