Make sure we always have a node to do IBD from

This introduces the concept of the 'sync node', which is the one we
asked for missing blocks. In case the sync node goes away, a new one
will be selected.

For now, the heuristic is very simple, but it can easily be extended
later to add better policies.

src/net.h

@@ -202,6 +202,7 @@ public:
     CBlockIndex* pindexLastGetBlocksBegin;
     uint256 hashLastGetBlocksEnd;
     int nStartingHeight;
+    bool fStartSync;
 
     // flood relay
     std::vector<CAddress> vAddrToSend;
@@ -242,6 +243,7 @@ public:
         pindexLastGetBlocksBegin = 0;
         hashLastGetBlocksEnd = 0;
         nStartingHeight = -1;
+        fStartSync = false;
         fGetAddr = false;
         nMisbehavior = 0;
         fRelayTxes = false;