Handle "conflicted" transactions properly

Extend CMerkleTx::GetDepthInMainChain with the concept of
a "conflicted" transaction-- a transaction generated by the wallet
that is not in the main chain or in the mempool, and, therefore,
will likely never be confirmed.

GetDepthInMainChain() now returns -1 for conflicted transactions
(0 for unconfirmed-but-in-the-mempool, and >1 for confirmed).

This makes getbalance, getbalance '*', and listunspent all agree when there are
mutated transactions in the wallet.

Before:
 listunspent: one 49BTC output
 getbalance: 96 BTC (change counted twice)
 getbalance '*': 46 BTC (spends counted twice)

After: all agree, 49 BTC available to spend.

src/wallet.h

@@ -700,8 +700,11 @@ public:
         // Quick answer in most cases
         if (!IsFinalTx(*this))
             return false;
-        if (GetDepthInMainChain() >= 1)
+        int nDepth = GetDepthInMainChain();
+        if (nDepth >= 1)
             return true;
+        if (nDepth < 0)
+            return false;
         if (!bSpendZeroConfChange || !IsFromMe()) // using wtx's cached debit
             return false;
 
@@ -717,8 +720,11 @@ public:
 
             if (!IsFinalTx(*ptx))
                 return false;
-            if (ptx->GetDepthInMainChain() >= 1)
+            int nPDepth = ptx->GetDepthInMainChain();
+            if (nPDepth >= 1)
                 continue;
+            if (nPDepth < 0)
+                return false;
             if (!pwallet->IsFromMe(*ptx))
                 return false;