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%
```

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);