addrman: use unordered_map instead of map

`CAddrMan` uses `std::map` internally even though it does not require
that the map's elements are sorted. `std::map`'s access time is
`O(log(map size))`. `std::unordered_map` is more suitable as it has a
`O(1)` access time.

This patch lowers the execution times of `CAddrMan`'s methods as follows
(as per `src/bench/addrman.cpp`):

```
AddrMan::Add(): -3.5%
AddrMan::GetAddr(): -76%
AddrMan::Good(): -0.38%
AddrMan::Select(): -45%
```

3 files changed, 36 insertions, 15 deletions

diff --git a/src/addrman.cpp b/src/addrman.cpp
index ceab1689d7..bc5d87e5f1 100644
--- a/src/addrman.cpp
+++ b/src/addrman.cpp
@@ -12,6 +12,8 @@
 
 #include <cmath>
 #include <optional>
+#include <unordered_map>
+#include <unordered_set>
 
 int CAddrInfo::GetTriedBucket(const uint256& nKey, const std::vector<bool> &asmap) const
 {
@@ -77,12 +79,12 @@ double CAddrInfo::GetChance(int64_t nNow) const
 
 CAddrInfo* CAddrMan::Find(const CNetAddr& addr, int* pnId)
 {
-    std::map<CNetAddr, int>::iterator it = mapAddr.find(addr);
+    const auto it = mapAddr.find(addr);
     if (it == mapAddr.end())
         return nullptr;
     if (pnId)
         *pnId = (*it).second;
-    std::map<int, CAddrInfo>::iterator it2 = mapInfo.find((*it).second);
+    const auto it2 = mapInfo.find((*it).second);
     if (it2 != mapInfo.end())
         return &(*it2).second;
     return nullptr;
@@ -408,8 +410,8 @@ CAddrInfo CAddrMan::Select_(bool newOnly)
 #ifdef DEBUG_ADDRMAN
 int CAddrMan::Check_()
 {
-    std::set<int> setTried;
-    std::map<int, int> mapNew;
+    std::unordered_set<int> setTried;
+    std::unordered_map<int, int> mapNew;
 
     if (vRandom.size() != (size_t)(nTried + nNew))
         return -7;
diff --git a/src/addrman.h b/src/addrman.h
index 41994288db..4929fd2ecf 100644
--- a/src/addrman.h
+++ b/src/addrman.h
@@ -8,22 +8,22 @@
 
 #include <clientversion.h>
 #include <config/bitcoin-config.h>
+#include <fs.h>
+#include <hash.h>
 #include <netaddress.h>
 #include <protocol.h>
 #include <random.h>
+#include <streams.h>
 #include <sync.h>
 #include <timedata.h>
 #include <tinyformat.h>
 #include <util/system.h>
 
-#include <fs.h>
-#include <hash.h>
 #include <iostream>
-#include <map>
 #include <optional>
 #include <set>
 #include <stdint.h>
-#include <streams.h>
+#include <unordered_map>
 #include <vector>
 
 /**
@@ -251,7 +251,7 @@ public:
 
         int nUBuckets = ADDRMAN_NEW_BUCKET_COUNT ^ (1 << 30);
         s << nUBuckets;
-        std::map<int, int> mapUnkIds;
+        std::unordered_map<int, int> mapUnkIds;
         int nIds = 0;
         for (const auto& entry : mapInfo) {
             mapUnkIds[entry.first] = nIds;
@@ -435,13 +435,13 @@ public:
 
         // Prune new entries with refcount 0 (as a result of collisions).
         int nLostUnk = 0;
-        for (std::map<int, CAddrInfo>::const_iterator it = mapInfo.begin(); it != mapInfo.end(); ) {
+        for (auto it = mapInfo.cbegin(); it != mapInfo.cend(); ) {
             if (it->second.fInTried == false && it->second.nRefCount == 0) {
-                std::map<int, CAddrInfo>::const_iterator itCopy = it++;
+                const auto itCopy = it++;
                 Delete(itCopy->first);
-                nLostUnk++;
+                ++nLostUnk;
             } else {
-                it++;
+                ++it;
             }
         }
         if (nLost + nLostUnk > 0) {
@@ -662,10 +662,10 @@ private:
     int nIdCount GUARDED_BY(cs);
 
     //! table with information about all nIds
-    std::map<int, CAddrInfo> mapInfo GUARDED_BY(cs);
+    std::unordered_map<int, CAddrInfo> mapInfo GUARDED_BY(cs);
 
     //! find an nId based on its network address
-    std::map<CNetAddr, int> mapAddr GUARDED_BY(cs);
+    std::unordered_map<CNetAddr, int, CNetAddrHash> mapAddr GUARDED_BY(cs);
 
     //! randomly-ordered vector of all nIds
     std::vector<int> vRandom GUARDED_BY(cs);
diff --git a/src/netaddress.h b/src/netaddress.h
index 897ce46cda..5c2d68150b 100644
--- a/src/netaddress.h
+++ b/src/netaddress.h
@@ -11,7 +11,9 @@
 
 #include <attributes.h>
 #include <compat.h>
+#include <crypto/siphash.h>
 #include <prevector.h>
+#include <random.h>
 #include <serialize.h>
 #include <tinyformat.h>
 #include <util/strencodings.h>
@@ -254,6 +256,7 @@ class CNetAddr
             }
         }
 
+        friend class CNetAddrHash;
         friend class CSubNet;
 
     private:
@@ -477,6 +480,22 @@ class CNetAddr
         }
 };
 
+class CNetAddrHash
+{
+public:
+    size_t operator()(const CNetAddr& a) const noexcept
+    {
+        CSipHasher hasher(m_salt_k0, m_salt_k1);
+        hasher.Write(a.m_net);
+        hasher.Write(a.m_addr.data(), a.m_addr.size());
+        return static_cast<size_t>(hasher.Finalize());
+    }
+
+private:
+    const uint64_t m_salt_k0 = GetRand(std::numeric_limits<uint64_t>::max());
+    const uint64_t m_salt_k1 = GetRand(std::numeric_limits<uint64_t>::max());
+};
+
 class CSubNet
 {
     protected: