Weight needs to be cached for OP_TXWEIGHT

[roconnor-blockstream]

The operation GetSerializeSize is O(transaction_size) instead of O(1). Thus this value need to be cached.

61b5152#diff-a0337ffd7259e8c7c9a7786d6dbd420c80abfa1afdb34ebae3261109d9ae3c19R2426-R2431

[apoelstra]

Closed by #1032