Remove vtxPrev from CWalletTx

vtxPrev was intended to make it possible to broadcast supporting transactions.

For various reasons it doesn't actually accomplish this goal while still consuming a sizable amount of space in the wallet.

I've removed most references to vtxPrev and replaced them with procedures that pull transactions from mapWallet.

As a side effect this code includes similar performance improvements to CWalletTx::IsConfirmed as https://github.com/bitcoin/bitcoin/pull/2952