Use snake case for blockhash

[tcharding]

Some Core C++ identifiers have snuck in. Snake case is more typical in this codebase.

gg 'blockhash: '                                                                                                                                                              ✘ 1 
client/src/client_sync/v23/blockchain.rs:17:            pub fn get_block_from_peer(&self, blockhash: BlockHash, peer_id: u32) -> Result<()> {
client/src/client_sync/v23/blockchain.rs:33:            pub fn get_deployment_info(&self, blockhash: &BlockHash) -> Result<GetDeploymentInfo> {
integration_test/tests/blockchain.rs:109:        prev_blockhash: node1.client.best_block_hash().expect("best_block_hash failed"),
integration_test/tests/mining.rs:138:            prev_blockhash: bt.previous_block_hash,
integration_test/tests/mining.rs:195:            prev_blockhash: bt.previous_block_hash,

Might be worth grepping for a few other common ones but this issue is explicitly only about blockhash.