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%
```

1 files changed, 19 insertions, 0 deletions

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: