Race condition in unit tests: blocks in initialization_blocks not cloned for each node

[dsiganos]

The way we apply nano::test::system::initialization_blocks to multiple nodes is technically wrong, although the problem is not easy to hit since the blocks are given to each node in sequence. But to be absolutely safe, we should clone each block before giving it to a node to ensure that 2 nodes do not work on the same block sideband at the same time.

To do this, we need a polymorphic way to clone blocks given a pointer to nano::block.

This affects unit tests only.