net: Use C++11 member initialization in protocol

Summary:
```
This change removes Init from the constructors and instead uses C++11
member initialization. This removes a bunch of boilerplate, makes the
code easier to read. Also, C++11 member initialization avoids accidental
uninitialized members.
```

Backport of core [[bitcoin/bitcoin#19020 | PR19020]].

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9190

src/compat/assumptions.h

@@ -52,6 +52,7 @@ static_assert(sizeof(double) == 8, "64-bit double assumed");
 //             code.
 static_assert(sizeof(short) == 2, "16-bit short assumed");
 static_assert(sizeof(int) == 4, "32-bit int assumed");
+static_assert(sizeof(unsigned) == 4, "32-bit unsigned assumed");
 
 // Assumption: We assume 8-bit bytes, because 32-bit int and 16-bit short are
 // assumed.

src/protocol.cpp

@@ -213,20 +213,6 @@ void SetServiceFlagsIBDCache(bool state) {
     g_initial_block_download_completed = state;
 }
 
-CAddress::CAddress() : CService() {
-    Init();
-}
-
-CAddress::CAddress(CService ipIn, ServiceFlags nServicesIn) : CService(ipIn) {
-    Init();
-    nServices = nServicesIn;
-}
-
-void CAddress::Init() {
-    nServices = NODE_NONE;
-    nTime = 100000000;
-}
-
 std::string CInv::GetCommand() const {
     std::string cmd;
     switch (GetKind()) {

src/protocol.h

@@ -420,14 +420,17 @@ static inline bool MayHaveUsefulAddressDB(ServiceFlags services) {
  * A CService with information about it as peer.
  */
 class CAddress : public CService {
+    static constexpr uint32_t TIME_INIT{100000000};
+
 public:
-    CAddress();
-    explicit CAddress(CService ipIn, ServiceFlags nServicesIn);
+    CAddress() : CService{} {};
+    explicit CAddress(CService ipIn, ServiceFlags nServicesIn)
+        : CService{ipIn}, nServices{nServicesIn} {};
 
     void Init();
 
     SERIALIZE_METHODS(CAddress, obj) {
-        SER_READ(obj, obj.Init());
+        SER_READ(obj, obj.nTime = TIME_INIT);
         int nVersion = s.GetVersion();
         if (s.GetType() & SER_DISK) {
             READWRITE(nVersion);
@@ -446,10 +449,10 @@ class CAddress : public CService {
         READWRITEAS(CService, obj);
     }
 
-    ServiceFlags nServices;
+    ServiceFlags nServices{NODE_NONE};
 
     // disk and network only
-    unsigned int nTime;
+    uint32_t nTime{TIME_INIT};
 };
 
 /** getdata message type flags */