Move CAddrDB frrom db to net

This was a leftover from the times in which
peers.dat depended in BDB.

Other functions in db.cpp still depend on BerkelyDB,
to be able to compile without BDB this (small)
functionality needs to be moved to another file.

src/net.h

@@ -690,4 +690,15 @@ class CTransaction;
 void RelayTransaction(const CTransaction& tx, const uint256& hash);
 void RelayTransaction(const CTransaction& tx, const uint256& hash, const CDataStream& ss);
 
+/** Access to the (IP) address database (peers.dat) */
+class CAddrDB
+{
+private:
+    boost::filesystem::path pathAddr;
+public:
+    CAddrDB();
+    bool Write(const CAddrMan& addr);
+    bool Read(CAddrMan& addr);
+};
+
 #endif