commitBatch and receiveBatch optimizations

[goran-ethernal]

This PR optimizes the commitBatch function on BridgeStorage contract, so that it doesn't use all the gas in block, since it saved the entire bridge messages contained in a batch. Now the function will receive a signed batch that contains:

struct SignedBridgeMessageBatch {
    bytes32 rootHash; // Merkle Tree root hash gotten from createing a merkle tree of all messages in a batch
    uint256 startId; // ID of first message in batch
    uint256 endId; // ID of last message in batch
    uint256 sourceChainId; // chain id of origin
    uint256 destinationChainId; // chain id of destination
    uint256[2] signature; // aggregated signature of signed batch
    bytes bitmap; // bitmap needed to verify the signature
}

Since the SignedBridgeMessageBatch now uses simple and fixed types, gas consumption dropped significantly, since we are not storing messages on the contract.

This PR also changes the receiveBatch function on Gateway contract, which the relayer will call to execute the batch. The function will now receive the list of brigde messages, signature and bitmap, and it will calculate the merkle tree root hash from messages, and check it against the signature.