Non-determinism hack: use raw file I/O for make_merge_file()
Fix race in PULSE counterexample Cg4 in PR 156.

Please see the PR 156 comment
https://github.com/basho/bitcask/pull/156#issuecomment-44251507
for details on the race condition fixed by this patch.

Fix: for the interest of safety, if we're iterating at all, then
an update will create a new sibling.