Fix de-serialization bug where AddrMan is corrupted after exception

* CAddrDB modified so that when de-serialization code throws an exception Addrman is reset to a clean state
* CAddrDB modified to make unit tests possible
* Regression test created to ensure bug is fixed
* StartNode modifed to clear adrman if CAddrDB::Read returns an error code.

src/net.h

@@ -779,6 +779,7 @@ public:
     CAddrDB();
     bool Write(const CAddrMan& addr);
     bool Read(CAddrMan& addr);
+    bool Read(CAddrMan& addr, CDataStream& ssPeers);
 };
 
 /** Access to the banlist database (banlist.dat) */