Make CAddrman::Select_ select buckets, not positions, first

Summary:
```
The original CAddrMan behaviour (before commit
e6b343d) was to pick a uniformly random non-empty bucket, and then pick a random element from that bucket. That commit, which introduced deterministic placement of entries in buckets, changed this to picking a uniformly random non-empty bucket position instead.

This commit reverts the original high-level behavior, but in the deterministic placement model.
```

Backport of [[bitcoin/bitcoin#23140 | core#23140]].

Depends on D12339.

Test Plan:
  ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12340

src/addrman.cpp

@@ -775,52 +775,75 @@ std::pair<CAddress, int64_t> AddrManImpl::Select_(bool newOnly) const {
         // use a tried node
         double fChanceFactor = 1.0;
         while (1) {
+            // Pick a tried bucket, and an initial position in that bucket.
             int nKBucket = insecure_rand.randrange(ADDRMAN_TRIED_BUCKET_COUNT);
             int nKBucketPos = insecure_rand.randrange(ADDRMAN_BUCKET_SIZE);
-            while (vvTried[nKBucket][nKBucketPos] == -1) {
-                nKBucket = (nKBucket + insecure_rand.randbits(
-                                           ADDRMAN_TRIED_BUCKET_COUNT_LOG2)) %
-                           ADDRMAN_TRIED_BUCKET_COUNT;
-                nKBucketPos = (nKBucketPos + insecure_rand.randbits(
-                                                 ADDRMAN_BUCKET_SIZE_LOG2)) %
-                              ADDRMAN_BUCKET_SIZE;
+            // Iterate over the positions of that bucket, starting at the
+            // initial one, and looping around.
+            int i;
+            for (i = 0; i < ADDRMAN_BUCKET_SIZE; ++i) {
+                if (vvTried[nKBucket]
+                           [(nKBucketPos + i) % ADDRMAN_BUCKET_SIZE] != -1) {
+                    break;
+                }
+            }
+            // If the bucket is entirely empty, start over with a (likely)
+            // different one.
+            if (i == ADDRMAN_BUCKET_SIZE) {
+                continue;
             }
-            int nId = vvTried[nKBucket][nKBucketPos];
+            // Find the entry to return.
+            int nId =
+                vvTried[nKBucket][(nKBucketPos + i) % ADDRMAN_BUCKET_SIZE];
             const auto it_found{mapInfo.find(nId)};
             assert(it_found != mapInfo.end());
             const AddrInfo &info{it_found->second};
+            // With probability GetChance() * fChanceFactor, return the entry.
             if (insecure_rand.randbits(30) <
                 fChanceFactor * info.GetChance() * (1 << 30)) {
                 LogPrint(BCLog::ADDRMAN, "Selected %s from tried\n",
                          info.ToString());
                 return {info, info.nLastTry};
             }
+            // Otherwise start over with a (likely) different bucket, and
+            // increased chance factor.
             fChanceFactor *= 1.2;
         }
     } else {
         // use a new node
         double fChanceFactor = 1.0;
         while (1) {
+            // Pick a new bucket, and an initial position in that bucket.
             int nUBucket = insecure_rand.randrange(ADDRMAN_NEW_BUCKET_COUNT);
             int nUBucketPos = insecure_rand.randrange(ADDRMAN_BUCKET_SIZE);
-            while (vvNew[nUBucket][nUBucketPos] == -1) {
-                nUBucket = (nUBucket + insecure_rand.randbits(
-                                           ADDRMAN_NEW_BUCKET_COUNT_LOG2)) %
-                           ADDRMAN_NEW_BUCKET_COUNT;
-                nUBucketPos = (nUBucketPos + insecure_rand.randbits(
-                                                 ADDRMAN_BUCKET_SIZE_LOG2)) %
-                              ADDRMAN_BUCKET_SIZE;
+            // Iterate over the positions of that bucket, starting at the
+            // initial one, and looping around.
+            int i;
+            for (i = 0; i < ADDRMAN_BUCKET_SIZE; ++i) {
+                if (vvNew[nUBucket][(nUBucketPos + i) % ADDRMAN_BUCKET_SIZE] !=
+                    -1) {
+                    break;
+                }
+            }
+            // If the bucket is entirely empty, start over with a (likely)
+            // different one.
+            if (i == ADDRMAN_BUCKET_SIZE) {
+                continue;
             }
-            int nId = vvNew[nUBucket][nUBucketPos];
+            // Find the entry to return.
+            int nId = vvNew[nUBucket][(nUBucketPos + i) % ADDRMAN_BUCKET_SIZE];
             const auto it_found{mapInfo.find(nId)};
             assert(it_found != mapInfo.end());
             const AddrInfo &info{it_found->second};
+            // With probability GetChance() * fChanceFactor, return the entry.
             if (insecure_rand.randbits(30) <
                 fChanceFactor * info.GetChance() * (1 << 30)) {
                 LogPrint(BCLog::ADDRMAN, "Selected %s from new\n",
                          info.ToString());
                 return {info, info.nLastTry};
             }
+            // Otherwise start over with a (likely) different bucket, and
+            // increased chance factor.
             fChanceFactor *= 1.2;
         }
     }