merge process should write CRC to hintfiles

[joecaswell]

Added calls to close_for_writing where merge process closes files so the CRC is properly appended to the hint file.

[dizzyd]

I understand the intent of this patch, but I think it would be better to make sure bitcask_fileops:close/1 does the right thing (i.e. adding CRC to hint file) when it's closing it. It's a slightly better patch but will make more sense to the reader and doesn't depend on knowledge of what's happening in a function so much.

[joecaswell]

bitcask_fileops:close/1 must be able to close both data files, which don't get a final CRC record, and hintfiles which do. I used bitcask_fileops:close_for_writing/1 to have the merge process use the same function as the vnode writer to accomplish the same task