Use wallet UTXOs whenever possible to avoid looping through all wallet txes

[UdjinM6]

This applies the same idea as #1655 to regular/non-PS wallet functions.

[codablock]

I'm wondering if this isn't a candidate for a helper method like this:

    struct WalletTxHasher {
        StaticSaltedHasher h;
        size_t operator()(const CWalletTx* a) const {
            return h(a->GetHash());
        }
    };
    std::unordered_set<const CWalletTx*, WalletTxHasher> GetSpendableTXs() const
    {
        std::unordered_set<const CWalletTx*, WalletTxHasher> ret;
        for (auto it = setWalletUTXO.begin(); it != setWalletUTXO.end(); ) {
            const auto& outpoint = *it;
            const auto jt = mapWallet.find(outpoint.hash);
            if (jt != mapWallet.end()) {
                ret.emplace(&jt->second);
            }

            // setWalletUTXO is sorted by COutPoint, which means that all UTXOs for the same TX are neighbors
            // skip entries until we encounter a new TX
            while (it != setWalletUTXO.end() && it->hash == outpoint.hash) {
                ++it;
            }
        }
        return ret;
    }

With this you can change all loops to something like:

for (auto pcoin : GetSpendableTXs()) {
  const uint256& wtxid = pcoin->GetHash();
  ...
}

There should be no overhead in this, as the compiler is able to optimize returning movable objects.

[codablock]

FYI, there was an error in the if (jt == mapWallet.end())...it did not increment it. EDIT Optimized it a little bit more.

[codablock]

utACK