net: CAddress & CAddrMan: (un)serialize as ADDRv2

Change the serialization of `CAddrMan` to serialize its addresses
in ADDRv2/BIP155 format by default. Introduce a new `CAddrMan` format
version (3).

Add support for ADDRv2 format in `CAddress` (un)serialization.

Co-authored-by: Carl Dong <contact@carldong.me>

src/streams.h

@@ -60,6 +60,7 @@ public:
     int GetVersion() const { return nVersion; }
     int GetType() const { return nType; }
     size_t size() const { return stream->size(); }
+    void ignore(size_t size) { return stream->ignore(size); }
 };
 
 /* Minimal stream for overwriting and/or appending to an existing byte vector