Change semantics of HaveCoinInCache to match HaveCoin

Previously it was possible for HaveCoinInCache to return true for a spent
coin. It is more clear to keep the semantics the same. HaveCoinInCache is
used for two reasons:
- tracking coins we may want to uncache, in which case it is unlikely there
would be spent coins we could uncache (not dirty)
- optimistically checking whether we have already included a tx in the
blockchain, in which case a spent coin is not a reliable indicator that we have.

src/coins.cpp

@@ -124,7 +124,7 @@ bool CCoinsViewCache::HaveCoin(const COutPoint &outpoint) const {
 
 bool CCoinsViewCache::HaveCoinInCache(const COutPoint &outpoint) const {
     CCoinsMap::const_iterator it = cacheCoins.find(outpoint);
-    return it != cacheCoins.end();
+    return (it != cacheCoins.end() && !it->second.coin.IsSpent());
 }
 
 uint256 CCoinsViewCache::GetBestBlock() const {