Merge #17785: p2p: Unify Send and Receive protocol versions

ddefb5c0b7 p2p: Use the greatest common version in peer logic (Hennadii Stepanov)
e084d45562 p2p: Remove SetCommonVersion() from VERACK handler (Hennadii Stepanov)
8d2026796a refactor: Rename local variable nSendVersion (Hennadii Stepanov)
e9a6d8b13b p2p: Unify Send and Receive protocol versions (Hennadii Stepanov)

Pull request description:

  On master (6fef85bfa3) `CNode` has two members to keep protocol version:
  - `nRecvVersion` for received messages
  - `nSendVersion` for messages to send

  After exchanging with `VERSION` and `VERACK` messages via protocol version `INIT_PROTO_VERSION`, both nodes set `nRecvVersion` _and_ `nSendVersion` to _the same_ value which is the greatest common protocol version.

  This PR:
  - replaces two `CNode` members, `nRecvVersion` `nSendVersion`, with `m_greatest_common_version`
  - removes duplicated getter and setter

  There is no change in behavior on the P2P network.

ACKs for top commit:
  jnewbery:
    ACK ddefb5c0b7
  naumenkogs:
    ACK ddefb5c0b7
  fjahr:
    Code review ACK ddefb5c0b7
  amitiuttarwar:
    code review but untested ACK ddefb5c0b7
  benthecarman:
    utACK `ddefb5c`

Tree-SHA512: 5305538dbaa5426b923b0afd20bdef4f248d310855d1d78427210c00716c67b7cb691515c421716b6157913e453076e293b10ff5fd2cd26a8e5375d42da7809d

src/net.h

@@ -831,7 +831,6 @@ public:
 
     std::deque<CInv> vRecvGetData;
     uint64_t nRecvBytes GUARDED_BY(cs_vRecv){0};
-    std::atomic<int> nRecvVersion{INIT_PROTO_VERSION};
 
     std::atomic<int64_t> nLastSend{0};
     std::atomic<int64_t> nLastRecv{0};
@@ -1018,6 +1017,7 @@ private:
     const NodeId id;
     const uint64_t nLocalHostNonce;
     const ConnectionType m_conn_type;
+    std::atomic<int> m_greatest_common_version{INIT_PROTO_VERSION};
 
     //! Services offered to this peer.
     //!
@@ -1037,7 +1037,6 @@ private:
     const ServiceFlags nLocalServices;
 
     const int nMyStartingHeight;
-    int nSendVersion{0};
     NetPermissionFlags m_permissionFlags{ PF_NONE };
     std::list<CNetMessage> vRecvMsg;  // Used only by SocketHandler thread
 
@@ -1069,16 +1068,14 @@ public:
 
     bool ReceiveMsgBytes(const char *pch, unsigned int nBytes, bool& complete);
 
-    void SetRecvVersion(int nVersionIn)
+    void SetCommonVersion(int greatest_common_version)
     {
-        nRecvVersion = nVersionIn;
+        m_greatest_common_version = greatest_common_version;
     }
-    int GetRecvVersion() const
+    int GetCommonVersion() const
     {
-        return nRecvVersion;
+        return m_greatest_common_version;
     }
-    void SetSendVersion(int nVersionIn);
-    int GetSendVersion() const;
 
     CService GetAddrLocal() const;
     //! May not be called more than once