fix bloom bitvecjournal storage allocation

[Hawstein]

Since the type of elem is u64, size/64 + extra is sufficient for the storage. Correct me if I am wrong or just miss something. Thank you:)

impl BitVecJournal {
	pub fn new(size: usize) -> BitVecJournal {
		let extra = if size % 8 > 0  { 1 } else { 0 };
		BitVecJournal {
			elems: vec![0u64; size / 8 + extra],
			journal: HashSet::new(),
		}
	}
// ... skip
}

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[Hawstein]

[clabot:check]

[parity-cla-bot]

It looks like @Hawstein signed our Contributor License Agreement. 👍

Many thanks,

Parity Technologies CLA Bot

[debris]

I think you are right! Thanks for contribution! 👍

[Hawstein]

@debris No problem:) So this PR can be merged?