Reduce time taken to collect VerifiedTransactions

[shawn-zil]

The original execute_block() collected verified transactions transactions from the Proposal block by re-verifying each transaction in the Proposal block. This triggers the recover_signer() library call for each transaction in the Proposal. It then fills up any missing ones from the mempool by cloning.

This PR flips this behaviour around to collect verified transactions first from the mempool, which already contains pre-verified transactions. It then fills up any missing ones from the Proposal block by only re-verifying those missing from the mempool.

This speeds things up by several orders of magnitude, when the transactions are already pre-verified and present in the mempool, with the worst case scenario being similar in timing to the previous behaviour e.g.

before

zilliqanode1      | 2024-10-04T01:31:05.720473Z  INFO zilliqa::consensus: 2635: 95 ELAPSED_VER 323.269721ms
zilliqanode0      | 2024-10-04T01:31:05.730972Z  INFO zilliqa::consensus: 2635: 95 ELAPSED_VER 323.800224ms
zilliqanode3      | 2024-10-04T01:31:05.756535Z  INFO zilliqa::consensus: 2635: 95 ELAPSED_VER 323.695312ms
zilliqanode2      | 2024-10-04T01:31:05.763893Z  INFO zilliqa::consensus: 2635: 95 ELAPSED_VER 325.994903ms

after

zilliqanode1      | 2024-10-04T02:17:40.488820Z  INFO zilliqa::consensus: 2653: 96 ELAPSED_INS 192.081µs
zilliqanode2      | 2024-10-04T02:17:40.510038Z  INFO zilliqa::consensus: 2653: 96 ELAPSED_INS 197.46µs
zilliqanode3      | 2024-10-04T02:17:40.519616Z  INFO zilliqa::consensus: 2653: 96 ELAPSED_INS 208.291µs
zilliqanode0      | 2024-10-04T02:17:40.790467Z  INFO zilliqa::consensus: 2653: 96 ELAPSED_INS 327.814331ms

The time taken to collect about ~95 VerifiedTransactions is about 325ms across all nodes (before) and is about 200us (if already present in the pool) to ~325ms (if not present in the pool).

As this is a small change that makes a measurable impact, with the worst case similar as before, I thought it would be good to merge this one first.

[shawn-zil]

Also impacts #1322 #1874