Chain - Benchmarks

[octavonce]

The following methods need to be tested on a Chain behind a ChainRef:

• [x] Chain::append_block() with a CheckpointBlock.
• [x] Chain::append_block() with an empty TransactionBlock.
• [x] Chain::append_block() with a full TransactionBlock.
• [ ] Chain::append_block() that flushes a new segment of a canonical chain to disk.
• [ ] Chain::append_block() on a forked non-canonical chain with an empty TransactionBlock.
• [ ] Chain::append_block() on a forked non-canonical chain with a full TransactionBlock.
• [ ] Chain::new() with a fresh chain.
• [ ] Chain::new() a longer chain saved on the disk.